The extract of a confidential note sent to Blair by attorney-general Lord Peter Goldsmith in March 2003, leaked to the British media, expressed some doubt as to whether a further UN resolution was needed to approve an invasion of Iraq.
To try to defuse the row, Blair released the attorney-general's advice on Thursday, something he had long refused to do.
Blair insisted he did not lie about the legal case for war in Iraq, but the opposition Conservative Party said the leaked document proved he did.
Foreign Secretary Jack Straw was dispatched within hours of the revelations late on Wednesday to do a round of television interviews, in an effort to control political damage to Blair's Labour Party, which is seeking a third consecutive term.
The full text of the memo, which had been leaked to Channel 4 news, was released on the prime minister's website.
"You have probably got it all anyway. I see no reason not to publish it," Blair said.
"You have probably got it all anyway. I see no reason not to publish it"
British Prime Minister Tony Blair
Conservative leader Michael Howard, who has branded Blair a liar, said the document reinforced doubts about Blair's integrity. "If you can't trust Mr Blair on the decision to take the country to war, the most important decision a prime minister can take, how can you trust Mr Blair on anything else ever again?" Howard asked.
Straw said Britain had nevertheless always wanted another resolution for political, if not legal, reasons.
"It was always preferable for there to be a second resolution, that was always our view, on political grounds, though not on legal grounds," he said.
"It was always preferable for there to be a second resolution, that was always our view, on political grounds, though not on legal grounds"
British foreign secretary
Straw, trying to calm the public furore that erupted after the leak on Wednesday, told the BBC that between 7 and 17 March, "circumstances" changed, which affected Goldsmith's advice.
He said new information had been presented in the interim to the UN Security Council detailing Iraq's breaches of past resolutions and that France had declared it would boycott a second resolution authorising military action.
British media linked the leak to the election campaign, with the rightwing Daily Express tabloid calling it "The big lie: final proof that Blair deceived the nation".
Many others contrasted the affair with the prime minister's "I have never told a lie" declaration made only hours earlier.
Blair, Howard and Liberal Democrat leader Charles Kennedy were due to appear together on a BBC panel programme later on Thursday.
The broadcaster said they would not debate one another but would answer audience members' questions separately.
With a week to go to election day, polls show Labour with a healthy lead.
A survey conducted by pollster ICM for Thursday's edition of The Guardian newspaper put Labour support at 40%, with the Conservatives at 33% and the Liberal Democrats at 20%. The margin of error was three percentage points.