Tony Blair considered quitting Number 10 and making a run for the European Commission presidency in 2004, according to Alastair Campbell.
The former communications chief makes the revelation in his latest diaries (2003 – 2005), serialised by the New European newspaper and published by Biteback.
Campbell, a close aide to Blair who quit Downing Street months after the 2003 invasion of Iraq, said the former prime minister approached then French president Jacques Chirac and German chancellor Gerhard Schroder about the potential bid.
But Blair, with two general election wins under his belt already, aborted the bid because he apparently felt rival and then chancellor Gordon Brown was attempting to push him out of power.
"A lot of the time the press exaggerated our difficulties," Campbell told the New European.
"This was one period where, if anything, they underplayed them because they didn't know just how bad things were.
"This was the closest Tony got to leaving and at the time I was terrified it would get out because it was one of those stories that would have taken on its own momentum.
"Tony had pretty much had enough and was being ground down by Gordon. In the end he realised that and decided he had to stay and see it though.
"Then came another on-off saga when he decided he was going to sack Gordon."
Blair went on to win his third general election with New Labour in 2005, later resigning from the top of British politics in 2007 to make way for Brown.
The new prime minister then failed to win the 2010 general election, with the Conservatives and Liberal Democrats forming a coalition government.
Portuguese politician José Manuel Barroso served as European Commission president between November 2004 and October 2014, with former prime minster of Luxemburg Jean-Claude Juncker succeeding him.
The Office of Tony Blair declined to comment at the time of publication.