His admission on Thursday that finding weapons of mass destruction in Iraq is not vital to justify the war dominated the front pages of Britain's newspapers Friday morning.
It is the first time that Blair has speculated on the possibility that WMDs will not be found.
In a speech to the US Congress, Blair said: "If we are wrong (and weapons of mass destruction are not found), we will have destroyed a threat that at its least is responsible for inhuman carnage and suffering. That is something I am confident history will forgive."
He added: "If we are right, as I believe with every fibre of instinct and conviction I have that we are, and we do not act, then we will have hesitated in the face of this menace when we should have given leadership. That is something history will not forgive."
However, many newspapers in the UK were scathing about the speech.
"Blair moves the goalposts" headlined the Daily Mail, a right-wing tabloid which supported the war, adding that "this was Blair the brilliant contortionist trying to have it both ways."
The Guardian said Blair's shift over weapons of mass destruction had been a "rare admission of fallibility" and a "significant softening" and "watering down" of Downing Street's stance on the issue.
And The Independent bemoaned that he did not use his address to point out the contradiction between America’s founding declaration - that 'all men are created equal' - and the reality of its attitudes to the outside world.
Blair left America for an official trip to Japan on Friday morning where he is likely to come under fresh fire about the WMD issue. His Japanese counterpart, Junichiro Koizumi, is also facing a domestic storm over his staunch support for the US and British-led war.
Meanwhile, US Democratic presidential candidate Bob Graham said on Thursday that there are grounds to impeach George W Bush if he is found to have led America to war under false pretences.
Bush is facing mounting questions over the failure to find conclusive proof Iraq had chemical, biological or nuclear weapons despite three months of occupying Iraq.
He is particularly under fire over a line in his January State of the Union address alleging that Saddam had sought uranium in Africa.
Tenet accepted blame for false
Graham said if the president lied about the reasons for going to war with Iraq it would be "more serious" than former President Bill Clinton's lie under oath about his sexual relationship with Monica Lewinsky.
"If the standard of impeachment is the one the House Republicans used against Bill Clinton, this clearly comes within that standard," he said.
CIA director George Tenet last week publicly accepted the blame for leaving the line in the speech even though his agency had repudiated it.
Niger uranium forgeries
It also emerged on Thursday that that a private source gave the US embassy in Rome forged documents detailing alleged Iraqi attempts to acquire uranium from Niger.
"In October 2002, we acquired these documents in Rome from a private source, a non-governmental source and they were immediately shared with all the appropriate agencies," a State Department official told Agence France Presse.
"The embassy shared them with all the relevant agencies at post and they were then shared again when they got back to Washington."
Italy has found itself at the centre of an international row over the now-discredited information.
The documents were used by the State Department two months after they were obtained in a fact sheet.
The Niger allegations then became part of the US campaign to convince the world community that Iraq was still pursuing a nuclear weapons programme, but were dropped after the CIA and other intelligence agencies in Washington cast doubt on their veracity.
Mohamed ElBaradei, the head of the UN's International Atomic Energy Agency revealed publicly in March that the documents had been forged.