Manchester United legend Roy Keane has revealed that he headbutted former team-mate Peter Schmeichel during a violent showdown in a hotel in Hong Kong.
The former Ireland international makes the stunning claim in his new autobiography The Second Half which is released on October 9.
In the book, the ex-Manchester United captain also speaks out about Rio Ferdinand's infamous missed drugs test in 2003 and also his battled with former boss Sir Alex Ferguson, who touched on the subject in his own autobiography last year - But the fight between Keane and Schmiechel is likely to grab the most headlines.
The pair formed the backbone of the most successful Manchester United side of all time, helping them to win five Premier League titles during their time together at Old Trafford.
Both also helped the Red Devils win the Champions League as part of the historic 1999 treble, although Keane was suspended for the final against Bayern Munich.
However it wasn't all trophies and smiles at Manchester United during the halcyon 1990's - with Keane claiming in his book that he never really got on with Schmeichel over what the midfielder perceived to be attention-grabbing behaviour.
The tension between the pair reached boiling point during a pre-season tour in Asia, where they drunkenly agreed to have it out.
Keane wrote: "I had a bust-up with Peter when we were on a pre-season tour of Asia, in 1998, just after I came back from my cruciate injury. I think we were in Hong Kong. There was drink involved.
"There'd been a little bit of tension between us over the years, for football reasons. Peter would come out shouting at players, and I felt sometimes he was playing up to the crowd: 'Look at me!'
"He was probably doing it for concentration levels, but I felt he did it too often, as if he was telling the crowd: 'Look what I have to deal with'.
"He said: 'I've had enough of you, It's time we sorted this out.' So I said 'Okay' and we had a fight. It felt like 10 minutes. There was a lot of noise - Peter's a big lad."
I woke up the next morning. I kind of vaguely remembered the fight. My hand was really sore and one of my fingers was bent backwards."
The full extent of the incident only became apparent in the morning when both had slept off the excess of the night before - with Nicky Butt referring the fight and filling Keene in on the details.
"The manager had a go at us as we were getting on the bus, and people were going on about a fight in the hotel the night before. It started coming back to me - the fight between me and Peter.
"In the meantime, Nicky Butt had been filling me in on what had happened the night before. Butty had refereed the fight. Anyway, Peter had grabbed me, I'd head-butted him - we'd been fighting for ages.
"At the press conference, Peter took his sunglasses off. He had a black eye. The questions came at him 'Peter, what happened to your eye?'"
[Schmeichel] said 'I just got an elbow last night, in training'. And that was the end of it.
Things then got even stranger when Ferguson started laying into his players - for waking up Manchester United legend Sir Bobby Charlton during the scrap.
"The first day back at the training ground, the manager pulled myself and Peter into his office.
"He knew exactly where we'd fought - I think he mentioned the 27th floor. He told us that we were a disgrace to the club, and that we'd woken Bobby Charlton up, that Bobby had come out of his room and seen us.
"Peter took responsibility for the fight, which was good. I admired him for it. But Sir Bobby could have tried to break it up."