Roy Keane has revealed that he once rejected the opportunity to sign for Real Madrid.

The Spanish giants made an approach for the Irishman following his departure from Manchester United in November 2005.

However, despite holding talks with Real's then sporting director Emilio Butragueno, Keane turned down a move to the Bernabeu and eventually joined Celtic.

Reflecting on that decision in his book The Second Half, Keane admitted that he perhaps made a mistake.

"Michael (Kennedy, lawyer) had given me a heads-up that Emilio Butragueno would be phoning, so I took my mobile phone everywhere with me. And, how's your luck, he rang me while I was sitting on the toilet. He said: 'Look Roy, we'll be glad to have you.' The club's board just had to sanction the deal," the 43-year-old wrote.

"I should have appreciated Real's offer more. It was the most attractive challenge in front of me but I didn't accept it. In hindsight, I should have said to myself: 'Go, go to Spain, live there for a year and a half, learn the language, learn the culture.'

"I took a negative approach. The weather and the training might have given me another lease of life, another two years of playing. As much as anything else, it was fear that decided me - fear of the unknown."

Keane made 13 appearances for Celtic before he retired in the summer of 2006 on medical advice.