Roy Keane has revealed he warned Alex Ferguson against taking on the Irish racehorse owner John Magnier and JP McManus in the Rock Of Gibraltar dispute that backfired on Manchester United and put in place the chain of events that led to the Glazer family's takeover.

Keane is heavily critical of Ferguson for pursuing the legal case and says he went to see the United manager to tell him he was taking on the wrong men and that it would have serious repercussions for the club.

Yet Ferguson ignored him and the dispute over stud fees for Rock Of Gibraltar, the retired racehorse, started to have damaging ramifications at Old Trafford, with Magnier and McManus using their position as major shareholders to submit their infamous 99 Questions document, predominantly looking at 13 transfers from the Ferguson era.

“Somebody I met in Ireland had told me to tell him [Ferguson]: ‘You are not going to win this,’ Keane writes in his new book, ‘The Second Half’. “I mentioned it to him. And I told him that I didn’t think it was good for the club, the manager in a legal dispute with shareholders.

“I felt I was entitled to say that. He was just a mascot for them. Walking around with this Rock Of Gibraltar – ‘Look at me, how big I am,’ – and he didn’t even own the bloody thing.”

Keane also reveals the details of his explosive departure from United in 2005, claiming a training ground bust-up with Ferguson and the assistant Carlos Queiroz made his Old Trafford exit inevitable. The former midfielder also detailed an incident where he head-butted Peter Schmeichel, leaving the goalkeeper with a black eye.

Keane’s shock departure was widely believed to be down to an interview with MUTV in which the Irishman criticised a number of his team-mates following a lacklustre performance in a 4-1 defeat by Middlesbrough. But Keane explains in the new book that a fierce falling out on a pre-season training camp on the Algarve proved the final straw.

“He was just on my right shoulder; how I didn’t fucking hit him again – I was thinking, ‘The villa in Portugal, not treating me well in training – and he just used the word “loyalty” to me,’” said Keane about Queiroz.

“I said, ‘Don’t you fucking talk to me about loyalty, Carlos. You left this club after 12 months a few years ago for the Real Madrid job. Don’t you dare question my loyalty. I had opportunities to go to Juventus and Bayern Munich.’ And while we’re at it we spoke about training downstairs. And were just on about mixing things up in training a bit.”

Keane went on to reveal that Ferguson soon stepped in, saying: “‘That’s enough. I’ve had enough of all this’,” which prompted the midfielder to round on his manager, replying: “You as well gaffer. We need fucking more from you. We need a bit more, gaffer. We’re slipping behind other teams.”

Manchester United fined Keane £5,000 for the MUTV interview but when Ferguson dropped him from a reserve game in which he was supposed to continue his rehabilitation from a broken foot, the Irishman knew the writing was on the wall. Keane claims that Ferguson and David Gill had prepared a written statement to confirm his departure and further angered him by getting the length of his service at Old Trafford wrong.

Keane continued: “I said to Ferguson, ‘Can I play for somebody else?’ And he said, ‘Yeah you can, cos we’re tearing up your contract’. So I thought, ‘All right – I’ll get fixed up.’ I knew there’d be clubs in for me when the news got out. I said, ‘Yeah – I think we have come to the end.’ I just thought, ‘Fucking prick’ – and I stood up and went ‘Yeah. I’m off.’”

Keane also said he regrets attempting to reconcile with Ferguson and Queiroz a few days later. “Now I kind of wish I hadn’t. Afterwards I was thinking, ‘I’m not sure why I fucking apologised.’ I just wanted to do the right thing. I was apologising for what had happened – that it had happened. But I wasn’t apologising for my behaviour or stance. There’s a difference – I had nothing to apologise for.”

Keane’s bust-up with Schmeichel came much earlier in his Manchester United career, on their pre-season tour to Asia in 1998, with the midfielder adding: “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.

“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 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?'"

Roy Keane: The Second Half, published by Weidenfeld and Nicolson, £20. www.orionbooks.co.uk.

(Guardian Service)