JOHN HARTSON'S Celtic career can be perfectly summed up as the ultimate sliding doors moment in Scottish football. It is well documented how the Welshman failed a medical at Ibrox with Rangers some 12 months before he joined Glasgow rivals Celtic.

According to the big man himself: "The best medical I have ever failed!"

The rest as they say is history.

Hartson signed for Martin O'Neill's Celtic on August 2, 2001, for the princely sum of £6 million from Coventry City after snubbing the advances of Steve McClaren at Middlesbrough. The Welshman made 201 appearances for Celtic in his five-year stint in Paradise scoring 109 goals in the process. He joined an elite band of 29 players who have hit 100 goals for Celtic.

Hartson revealed that it was Gordon Strachan who played a pivotal role in him moving north of the border.

READ MORE: Former Celtic assistant John Robertson on Martin O'Neill, Seville and bringing success back to Parkhead - The Big Interview

Hartson said: "Coventry City had just been relegated and the club couldn't afford to keep me, Mustapha Hadji and Craig Bellamy on the books. The chairman Brian Richarson and Gordon did everything to get me to stay at the club.

"They offered me a huge bonus as a signing on fee and Gordon offered me the captaincy. I wanted to play in the top flight in England but as it happened I came to Celtic.

"Gordon told me he had Martin O'Neill on the phone and he advised me to sign for Celtic as I would score lots and lots of goals. I had heard about the size of the club and wee Gordon helped sway my mind really."

Celtic Way:

Hartson then proceeds to tell a comedic and cautionary tale of how Celtic's majority shareholder Dermot Desmond used a ruse to get targets to sign on the dotted line.

Hartson joked: "I was at Heathrow airport waiting to fly to Glasgow to sign for Celtic and to meet Martin O'Neill. My agent said to me, 'John, there is somebody on the phone wanting to speak to you.' It was Dermot Desmond.

"Dermot said to me that he was delighted that I had opted to sign for Celtic and they were pleased to get the deal over the line. Martin has been chasing you for a while and he is happy to finally get his man. That'll be great.

"I told him how excited I was and that I couldn't wait to get going at Celtic. Dermot then said to me, 'Do you like golf, John? Have you ever heard of Tiger Woods? Do you fancy a round of golf with Tiger Woods?'

"I was a keen golfer, I loved golf. I couldn't believe this was happening. I said to Dermot, 'What? A round of golf with Tiger Woods? Are you serious? Aye, bloody right I would.'

"I am still waiting on that phone call from Dermot and to play that game of golf with Tiger Woods to this day! Tiger Woods was never mentioned again. I could see exactly why Celtic enticed players to sign for the club in the Martin O'Neill era."

Hartson readily admits that he had no allegiances when it came to Scottish football and that he would have signed for Rangers had they not spotted something in his knee that gave them cause for concern. The Light Blues' loss was Celtic's gain.

Hartson said: "My club was Swansea and if I had to say I followed and supported any other team it would have been Liverpool as I played alongside Ian Rush with Wales and he was my icon and football hero growing up. Since then it has all changed and I have the Celtic crest tattooed on my arm.

"I was going to sign for Glasgow Rangers. I failed many medicals in my career - at Spurs, Charlton and Rangers. I couldn't pass a medical because of my knee.

"The Rangers medical was the best one I ever failed in my life! Martin assured me that I was signing for Celtic regardless. He uttered that legendary line: "John unless you have a hole in your heart, I will sign you for Celtic."

READ MORE: Facing Henrik Larsson: Celtic's magnificent seven as seen by the defenders he tormented

Hartson had no problem putting pen to paper. His next dilemma was how he was going to replace the dynamic strike duo of Chris Sutton and Henrik Larsson who had helped guide Martin O'Neill's men to their first domestic clean sweep since 1969.

He duly got his chance and endeared himself to the Celtic faithful when he bagged a hat-trick against Dundee United in a 5-1 win.

Hartson said: "My thoughts when I signed for Celtic was how am I going to break these two up? How am I going to dislodge Larsson or Sutton from this Celtic team?

"They had played so well and guided Celtic to the treble the season before. Not since the great Jock Stein had Celtic reached such domestic heights. It was a masterstroke by Martin O'Neill bringing in the likes of myself as he did not want to stand still.

"My first goals came in a hat-trick at home against Dundee United after a few matches without scoring and I stayed in the team ever since. I never came out of the team for a period of five years unless I was injured. Chris played in midfield and at the back at times for Celtic as Martin found a way to accommodate us all in the team.

"If you cannot work alongside Chris Sutton and Henrik Larsson then there is something wrong with you. Henrik Larsson was a phenomenon.

"He would run in behind, he had a great leap, he had great football intelligence, he was a great finisher and technically he was magnificent. He was a wonderful athlete. Henrik made playing football look easy at times. Both Chris and Henrik made me a better player.

"If you'd flick the ball in and it wasn't a good one either Henrik or Chris would read it and turn it into a good one and they would work off your touch. It was just an honour, a pleasure and a privilege to play alongside players of that calibre."

By 2003, Hartson is hitting his stride at Celtic. He bags 25 goals in a memorable campaign that sees Celtic reach the UEFA cup final in Seville. A back injury robs him of the chance to play in the final against Porto. However, Hartson enjoys some special moments along the way.

He nets the vital away goal in Spain during a 2-1 defeat against Celta Vigo that keeps Celtic in Europe beyond Christmas for the first time since 1980 when they lost in the European Cup quarter-final to Real Madrid. That same night Celta Vigo's Peter Luccin gobs on him and Hartson is blissfully unaware of it. He only finds out about the disgusting incident on his return to Glasgow airport.

Hartson said: "The year 2003 is a bitter-sweet one for everybody. Reaching Seville was fantastic but a back injury robbed me of the chance to play in the UEFA cup final.

"I scored 25 goals that season and I was fouled by Lorenzo Amoruso and my back had gone during "Beachball Sunday" at Ibrox when we won 2-1. There were six games to go.

"I had burst the disks in my back and the day after that game my dad and my wife had to carry me down the stairs. I knew then I wasn't going to play in the UEFA cup final.

READ MORE: Anatomy of a Celtic goal: John Hartson on his incredible Anfield strike on the road to Seville

"I wanted to play in the league games more than anything and I wanted to win the title. Everybody was getting carried away with the UEFA cup final but I wanted to win the league and score over 30 goals so I was just as upset at that. Many Celtic supporters tell me that we would have won the UEFA cup had I played that night.

"We will never know but it is very complimentary to my talents as a player. Ironically Martin has said that Paul Lambert - a Champions League winner with Borussia Dortmund - would have missed out that night as he would have started with Henrik and me in the attack.

"It was a ridiculously hot night in Seville and I would have had blisters on my eyelids had I played!

"I enjoyed some wonderful moments during that UEFA cup run. I scored the goal against Celta Vigo that guaranteed Celtic European football beyond Christmas for the first time in 23 years. That was such an important goal for the club in terms of finance and prestige.

"I stuck my backside into their centre-half and cracked home a low shot. Another great Celtic player used to use his backside like that! (He is referring to King Kenny Dalglish).

"I was so chuffed at that but all hell broke loose when I got home as I wasn't aware that Peter Luccin had spat on me. It was only when I got to Glasgow airport and the press pack were seething and they were trying to get a reaction out of me and they asked me all about it.

"That was the first time I was aware of the incident. I was shouting in the airport, 'That bastard, can we play the game again?' It is utterly disgusting and thankfully I didn't react on the park as I don't know how I would have reacted as I might have stiffened Luccin and got sent off. When somebody spits on you that is the worst thing that a fellow professional can do to a fellow player."

Celtic Way:

Hartson also smashes home a superb solo clinching second goal at Anfield in a 2-0 victory against Liverpool in the UEFA cup quarter-final days after missing a last-minute penalty against Rangers in the League cup final at Hampden which hands the trophy to their rivals.

It's a joyous night for Hartson as his family are all there to see him in action and he admits to being "kissed by an angel" especially after being roused by a stirring Martin O'Neill team talk that has entered into Celtic folklore.

Hartson said: "Henrik had burst his jaw against Livingston but he returned for the League Cup final against Rangers. Whilst Henrik was out of the team Celtic had got two penalties and I had scored them both.

"We get a penalty at Hampden and it is the last kick of the game. I had a terrible decision go against me when I scored a perfectly legitimate goal and the linesman flags for offside. I couldn't believe it.

"We get the penalty and Henrik chucks me the ball and just says: "John, take it. What a professional he was. I should have said no to Henrik as he was the penalty taker at the club and I should have handed him the ball back.

"I was confident I'd score so I hit and Stefan Klos dived to his left and I put it to the inside of his right post. I pulled it slightly and it drifted wide of the post. That was a real low moment for me at Celtic.

"I genuinely felt I had let everybody down and that's the initial feeling I had. I lasted 30 minutes when we got back to Celtic Park without the trophy as I felt like shit and I was so low. We trained the next day before heading to Anfield and we were all subdued.

"If I had scored that penalty it would have made it 2-2 and we had recovered from 0-2 down and momentum was with us. We would have won the cup in extra time. I am entirely convinced of that.

"Chris wasn't available for Anfield but we go there and I hit the wonder goal of a lifetime. It is the best goal I have scored in my life. I smashed it from 30 yards and it flew into the top corner. I was kissed by an angel that night.

"I had an angel on my shoulder as my mum never came to any games but she travelled up because we had family in Bangor in North Wales. She was in the stands behind the goal in the away end at Anfield against Liverpool.

"My mum still says to me to this day, 'John, if the net wasn't there I would have caught the ball!' I still get goosebumps thinking about what my mum said to this day.

"What are the chances that my mum would be right behind that goal as it soars into the net? It's just wonderful really.

"I'd gone from feeling so low in the League cup final to scoring the goal that put Celtic into the UEFA cup semi-final. Those are the defining moments in your career and football. The ones that encapsulate the game.

"The other thing I remember about Anfield was Martin O'Neill's team talk beforehand. It was magical. He put it all on the line.

"Martin O'Neill basically said to the Celtic players, 'Nobody comes here and wins. It's Anfield. They've got five European Cups and under the lights, at Anfield, they beat everybody. We might as well fucking pack up and go home now unless you lot are willing to go out and scrap for this. So let's make sure they don't fucking beat us! We've got a draw and no matter what happens tonight we have to score a goal. If we don't score a goal then we are out. They have got the away goal with Emile Heskey. Let's get the ball into big John and Henrik and see where it takes us.'

"It was legendary stuff and the hairs on the back of the neck stood on end. He told the starting eleven that night, 'Look at the player next to you. Can you trust him?

"It was all that stuff. He would say, 'John look at Thommo, Thommo, look at Henrik, Henrik, look at Lenny. Look at the player sitting next to you. He'd dig you out of the shit - wouldn't he? He'll run for you, he'll do anything for you. Will you do the same for him?'

"It was so inspiring. After a team talk like that how could we lose?

"We couldn't lose the game as Martin would have dropped us all off from the team bus on the motorway and told us all to walk home! Martin was a force of nature and he was absolute class.

"I loved him as a manager as he was fantastic. I idolised the man because I had failed four medicals before I came to Celtic. Not a lot of people would have touched me with a barge pole back then but Martin did.

"I signed and I did not put one pack of ice on my knee in five years playing for Celtic! I can't go anywhere today and I mean anywhere, I have been to Bermuda, I have family in Newfoundland in Canada, you name it, I will bump into a Celtic fan.

"That is all down to Martin O'Neill as he wanted to make me a Celtic player. It makes you feel brilliant. I have real empathy with the supporters and I always stand around and sign autographs. It is nothing for me to do that.

"I can't imagine what it would be like being a schoolkid and if Liverpool were playing and Ian Rush walked past me without posing for a picture or signing an autograph. I knew you can't sign two thousand autographs but you do as many as physically possible.

"There are only 29 players who have scored 100 goals for Celtic. That's an exclusive list and you will forever be remembered.

"I wanted to make that impact at Celtic. I am not a Henrik Larsson or a Jimmy Johnstone or a Kenny Dalglish or a Lisbon Lion as they are Celtic greats.

"I just wanted to do well for the club and make a fist of it. To score over 100 goals for Celtic is really special."

Celtic Way:

The former Arsenal, Wimbledon, Coventry and Wales striker - he scored 14 goals in 51 internationals for his country - won three Scottish Premiership titles, two Scottish Cups and a Scottish League Cup with Celtic.

He also scoops the PFA Scotland Players' Player of the Year alongside the Scottish Football Writers Association Footballer of the Year in 2004/05. The latter gong he shared with the late Rangers star Fernando Ricksen.

Hartson said: "I was delighted to win both those individual awards. For the life of me I still don't understand how we shared the players' award as surely Fernando Ricksen - god rest his soul - had one more vote than me or vice-versa.

"It was a real tragedy and shame what happened to Fernando. It was very sad. We shared the PFA Scotland trophy for six months each. It was just a real honour for me to win the awards."

Hartson's five years in Paradise are awash with golden memories.

He recalls how Martin O'Neill's side took on the cream of European and English football and rarely ended up empty-handed.

READ MORE: Paul Lambert recalls how Martin O'Neill's Celtic side humbled Ajax in the Champions League qualifiers

The Welshman insists that the Celtic players were hurt by spiteful comments that slurred Scottish football.

Hartson said: "The night I signed for Celtic we played Manchester United at Old Trafford in Ryan Giggs' Testimonial. It was a full-house and Celtic put in a performance.

"Martin O'Neill told the players this wasn't an exhibition match and Celtic beat them 4-3.

"Lenny and Sutty scored and we ran riot. Manchester United did not lose many games back then and Celtic were magnificent.

"We beat Blackburn Rovers, Liverpool, Juventus, Porto, Lyon, Ajax, Anderlecht, Valencia and turned them all over. The Scottish league is seen as a weaker competition and it was supposedly for players in their 30s to sign for one of the big two and have a bit of a dawdle before winding down their careers.

"That used to hurt us. We had a team crammed full of international players.

"We had Chris Sutton who'd won a golden boot and signed for Chelsea for £10 million. We had Paul Lambert who was a Champions League winner. I had played in attack with Denis Bergkamp and Ian Wright. Neil Lennon had played over 200 games for Martin O'Neill before he signed him. We had the genius that was Henrik Larsson. We had Alan Thompson who had played for big clubs like Newcastle and Bolton. We had Stan Petrov who was one of the best 25-year-old players in Europe.

"We had bloody good players and they were quality. How do you teach Henrik how to score a goal? You don't. How do you tell Chris Sutton how to bring a ball down on his chest? You can't.

Celtic Way:

"Graeme Souness never ever lived that 'men against boys' comment down when we played Blackburn Rovers in the UEFA cup. We went down to Blackburn and played them off Ewood Park when Henrik and Chris both scored.

"The only way we could respond was to beat any opposition. Martin O'Neill used to tell us that he fancied us against anybody. We rarely held team talks as Martin O'Neill trusted every one of his players. Celtic won many games in that era because we had such a good side.

"The one blip in the Martin O'Neill era was "Helicopter Sunday."

Hartson admits that even now those who were involved in the blowing of the title on the last day of the 2004/05 season cannot bring themselves to talk about it.

Two late Scott McDonald goals crushed Celtic hearts at Fir Park and condemned them to a 2-1 defeat as Rangers won the league on a last-day cliffhanger courtesy of a 1-0 win at Easter Road.

He said: "That was a huge disappointment when you consider that the front three of Celtic that day was me, Chris Sutton and Craig Bellamy. We thought we had it in the bag.

"It is the one time I can honestly say we took our eye off the ball and maybe got complacent. We failed to wrap up the game. A second goal would have killed it and we would have won the game and the championship.

"My dad was there that day at Fir Park and I was thinking where was I going to run as I was ready to celebrate. In the blinking of an eye, we didn't only lose the game we had lost the league. It is the worst I have probably seen Martin O'Neill.

"He didn't say anything. You could have heard a pin drop. He had his head in his hands and his elbows on his knees and he just said let's grab a shower and get out of here. That was all he did.

"It was the lowest I have ever seen him even after the UEFA cup final. I won three titles out of five in Scotland and I lost the other two by a goal and a point. How tight is that to five out of five?

"I could easily be talking about winning five out of five league titles in Scotland. It is one of those days we got complacent yet you are talking about top players here.

"I have never spoken to any of my former teammates about Helicopter Sunday. I did something for Sky Sports a couple of years ago but I have never spoken to the Celtic players about it.

"I have never mentioned it to Martin O'Neill. It is the great unmentionable and it is buried in a box somewhere.

"Nobody could foresee what happened that day. Teams can lose any football match. Celtic always stepped up on those occasions especially when there was so much on the line.

"Ironically I scored 30 goals for Celtic that season but it was not to be. That was the one and only time I hit 30 goals in a campaign."

Celtic Way:

Twelve months later Hartson would score the goal that sealed the title for Celtic against Hearts and whilst everything appeared perfect on the park, it was a train-wreck off it for the hitman as he was in the midst of a divorce as well as developing a much-publicised gambling habit.

Hartson said: "The following season I scored the goal that clinched the title against Hearts in Gordon Strachan's first season. I enjoyed that moment especially as I had a feeling that I was going to leave the club at that time.

"I was going through a painful divorce and it was playing on my mind. It was the break-up of a family unit that was horrendous. I couldn't handle training and coming home and not having my kids in my life. It hurt.

"I was living on my own in the West End. I wanted to pick my kids up from school and then I would remember that we were separated and it was killing me. My kids were two years old and six years old. It was not a nice period.

"I was experiencing a human emotion and the Celtic supporters were blissfully unaware of it. Somehow come the weekend I had to flick that footballer switch and try and perform. A divorce is one of the worst things you can go through mentally.

"The break-up of the family and the realisation that it is over. You can't sit at the table with your kids and see them when they come home from school - it's horrible.

"I was suffering. I had a gambling addiction to and I was gambling a lot back then. It was all weighing heavy on my mental health. I have not had a bet in 10 years though.

"Gordon Strachan made the decision for me as he brought in Kenny Miller and Scott McDonald and I knew my time at Celtic was up by then.

"I ended up signing for West Bromwich Albion under Bryan Robson."

Celtic Way:

In 2008, two years after leaving Celtic, Hartson then faced up to the biggest battle of his career - cancer. Thankfully it failed to claim him.

Hartson has since gone on to raise over £1 million for cancer charities owing to setting up the John Hartson Foundation. He is rightfully proud of those achievements, deriving more personal satisfaction from that more than any trophy or medal could convey.

However, it was ex-Celtic chief-executive Peter Lawwell and his former club who stepped up to the plate and came to the fore when he needed them most as he was recovering from the disease.

Celtic Way:

Celtic has always been a constant in Hartson's life.

Hartson said: "Two years after leaving Celtic, I got cancer and had that battle to deal with. The surgeons saved my life and I was in a bad way at that time.

"Not many people know this but Peter Lawwell phoned my dad every day I had cancer. This is a side to Peter that the Celtic supporters don't get to see. He told my dad not to worry about anything and that any treatment I needed, Celtic would provide it.

"Celtic said they would look after for any after-care treatment that I needed. Peter went above and beyond my estimation of a human being.

"The hospital had never seen anything like it in Swansea. The institution couldn't cope with that level of intrusion, it was crazy as tabloid newspapers were outside trying to get photos of yours truly. Loads and loads of boxes, cards, letters and gifts would arrive on a daily basis and it all helped.

"The outpouring of love and support for me was really overwhelming. I also have to say that Tottenham Hotspur fans, Cardiff City fans and Rangers supporters got in touch with me and they were absolutely incredible.

"I cannot thank them enough for what they did at that time. People that used to boo me were now cheering me on in my biggest ever battle - for survival. How humbling is that?

"The proudest thing I have achieved in my life is raising over £1 million for cancer charities through the John Hartson Foundation. I will be presenting a cheque for £50,000 to a breast cancer charity soon.

"When I walk into where the Glasgow offices are and I see those women raising money for charity every day of their lives because they receive no funding and they rely on people doing events for them that for me is bigger and better than scoring any goal in any league in any country. That money is helping save people's lives.

"Every penny the John Hartson Foundation raises we give away. The charity has gone from strength to strength. I still chair Gamblers Anonymous meetings and I have one coming up in Airdrie soon.

"I go for tea and biscuits and I always take something out of the meetings. It is like therapy for me."

So how would Hartson sum up his five years at Celtic?

Hartson said: "I feel lucky and blessed to have played for Celtic. It's a very special club and it was an honour to play for them and represent those wonderful supporters."

Big Bad John.

The very nickname is a total misnomer. He does not have a bad bone in his body.

The world would be a far better place if there were more John Hartson's in it.

He is a gentle giant.