The former prime minister’s pledge, which was approved by the then US president, is disclosed in a Downing Street memo leaked to the Daily Mail.
It was written by Mr Blair’s foreign policy chief Sir David Manning one day after Mr Blair’s summit with Mr Bush at the president’s ranch in Crawford, Texas, in April 2002, a year before the conflict.
It was not until two years after the war that evidence emerged suggesting Mr Blair had ‘signed in blood’ at Crawford a deal to back Mr Bush’s plan to bomb Iraq.
But evidence of Mr Blair’s role in manipulating public opinion was clear much earlier, with his press chief Alastair Campbell clashing with the BBC when it accused the Government of ‘sexing up’ the case for war.
The fight was over the notorious ’45-minute dossier’, which was spun to newspapers as suggesting Saddam Hussein could hit British targets in Cyprus with weapons of mass destruction within 45 minutes of an order by the Iraqi dictator.
Sir David’s memo also shows that Mr Blair made the decision to back the war – almost an entire year before it began – after Mr Bush blithely told him he did not know who would replace Saddam and wasn’t bothered about that.