Mr Lynam, 55, who has been with the BBC for 30 years, is thought to have accepted a four-year contract worth pounds 5m to present ITV's coverage of the European Champions' League and the FA Cup.
The BBC was only told of Mr Lynam's move yesterday when he informed the corporation's deputy director-general, Will Wyatt. Mr Lynam said he did not give the corporation a chance to make a counter-offer because his mind was made up.
Corporation insiders said lawyers would be examining the presenter's contract which was due to run until next year.
Mr Lynam said he was looking for a new challenge: "This is quite a monumental day for me. The reasoning behind this move is I needed a change - a breath of fresh air," he said.
The presenter declared that he did not believe BBC sport to be in decline, despite the corporation's recent loss of high profile events. However it seems clear that ITV's pounds 250m acquisition of the television rights to Champions' League football for the next four years swung his decision to move.
David Liddiment, ITV's director of programmes, said: "It was a happy coincidence that we had the four-year contract when he was thinking of a change."
Bob Shennan, the BBC's head of sport, put a brave face on the news: "There is no question that Des leaving us is disappointing. But I think he would be the first person to say that no presenter is bigger than BBC Sport as a whole."
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