A group of US congressmen has held an informal hearing into a memo that suggests President George Bush decided on the Iraq war months in advance.
Mr Blair and Mr Bush deny the war was planned so far in advance
More than 100 Democrats took part in the public forum, calling on the White House to explain the leaked UK memo.
Although the hearing in Washington was unofficial, a BBC correspondent says it coincides with waning public support for the war in Iraq.
President Bush denies he had made up his mind at the time to attack Iraq.
The British Sunday Times newspaper published the so-called Downing Street memo, dated 23 July 2002, on 1 May, after it was leaked by a former UK foreign policy aide.
In the memo, British Foreign Secretary Jack Straw is quoted as saying Mr Bush had made up his mind to take military action even if the timing had not yet been decided.
A second memo, published by the paper this week, says UK ministers were told that they had no choice but to find a way to make the war in Iraq legal.
Scant US coverage
BBC correspondent James Coomarasamy says the congressmen want to raise awareness of what is a slow-burning political issue.
The affair has received scant coverage in the mainstream US media, although left-wing bloggers have had some success in bringing it to public attention.
"Our investigation is just commencing and I can tell you more and more people are getting disturbed, some outraged, by what these disclosures suggest has happened," Michigan Congressman John Conyers told the Associated Press news agency.
Mr Conyers has urged Republicans to take part in the House of Representatives' hearing, though none appears to have agreed to do so.
However, around 500,000 Americans have signed a petition against the war.
Gold Star Families for Peace, a non-partisan group, has expressed its support for the hearing.
"We want our Congress to stand up, to identify and investigate the lies and follow it wherever it goes," Celeste Zappala, who lost a son in Iraq, told AP.
Reg Keys, who has campaigned against the war in the UK since his son was killed in Iraq, is expected to give evidence at the hearing and address a public rally outside the White House.
'Peaceful solution sought'
The hearing comes as polls suggest nearly three-quarters of Americans view the US casualty rate in Iraq as unacceptable.
For the first time, a majority appears to believe that the war has not improved long-term US security.
Mr Bush says that at the time of the memo he was prepared to find a peaceful way to deal with former Iraqi leader Saddam Hussein.
"Somebody said we had made up our mind to use military force to deal with Saddam," he said.
"There is nothing further from the truth. My conversation with the prime minister was 'How can we do this peacefully?'"
White House officials say the president is now planning to sharpen his focus on Iraq, but there is no suggestion that this will involve any policy change.
Too little, too late. Now that we have started this thing we are doomed to another Afghanistan (if we leave too early) or another Vietnam (if we don't leave soon). And how many deaths are acceptable if the current number are too many? And why did anyone think this was going to make Americans safer at home?
Caitlin, Boston, MA, USA
Regardless of whether the public was deceived or not, it's for the good of the country and we should support the President. Our enemies are watching all of this gleefully, plotting their next attack.
Jesse M. Knudsen, Brooklyn, NY
Now that the cat is out of the bag the "scant U.S. media coverage" hopefully should heat up. The whole world knew that there was no WMDs except for the dynamic duo of Bush and Blair. What is the saddest thing of all is that thousands of innocent Iraqi men, women and children as well as soldiers from the "coalition of the willing" lost their lives for this lie. This is the most unkindest cut of all and unforgivable.
John Dawes, Sydney, Australia
"The hearing comes as polls suggest nearly three-quarters of Americans view the US casualty rate in Iraq as unacceptable. " I wonder what their opinion is on the number of Iraqi casualties, that is if they've been told....
Stuart Goodall, Bristol, U.k
No one is really surprised to learn of this 'revelation'. What is surprising is how little to no effect the fact that the UK and the USA arm-in-arm decided to illegally go to war, is having on these governments. Iraq illegally went to war against Kuwait, the world responded with outrage, sanctions and ultimately military action. Should the USA and UK not be condemned or treated in the same way. And then you still wonder why this 'double standard' makes terrorists so determined in their cause?
Yet another revelation - it all proves that, however well-meaning Mr Bush may be in his own mind, he and his administration have, in my view, clearly engaged in a simple con-job to justify action fundamentally in breach of international law. The pity is that this cynical war has severely damaged the US itself (in several ways) and the final outcome is likely to create more and worse problems globally than existed previously. No nation, however powerful militarily and economically, can ever really get away with an assault on the sovereignty of another state. And it's all so obvious from history!
Alfred P Zarb, Leura, Blue Mountains - Australia
At last the American public is beginning to see through the smoke screen. I can't believe how long it has taken! They are so often too easily led by the US Gov. Coupled with no coverage of anything anti-war due to media suppression and you can't be surprised at the outcome I suppose. Thankfully we have some impartial news coverage here in the UK. Although even a march of millions on London didn't stop a terrible wrong being done. Let's hope the American public now ask some serious questions on other issues. Perhaps the reasons behind the entirely industry-led US government's stance on climate change!
Duncan, mid-Wales, UK
Bush and the neo cons came to power with the oil and strategic merits of Iraq at the top of their agenda. 9/11 allowed them to hoodwink a ill-educated and misinformed population on both sides of the Atlantic into believing we needed to invade Iraq. Anecdotal evidence has revealed that the meetings held in the immediate aftermath of 9/11 focused on a single objective: linking Iraq to 9/11. The existence of this memo is no great revelation. The real scandal here is the ignorance of the American media to these documents. This once great institution, now led, in my view, by right wing propaganda merchants, is failing its citizens on every possible level. The most powerful country in the world is also the least informed, and its media is to blame.
James Weston, UK
Of course the decision to invade Iraq was made a long time prior to the invasion. Whilst in Australia there was a newspaper report after the war of how the Australian SAS were already inside Iraq attacking key defence installations prior to the "Date of Invasion". If they were in there before the invasion then the decision to invade must have already have been made some time beforehand. Was there any point in the peaceful protests no - as the decision was already made! In my view, there is no democracy left in the Western World!!
David Waterston, Sheffield
Many American have long known that pre-emptive war was Bush's style. Iraq was on Bush's agenda in 2002 as you have revealed. It has always been subliminal in his awkward political rhetoric. Unfortunately most mainstream US media outlets have been intimidated and have bowed to the right wing 'state of siege' mentality and have refused to carry anything but jingoist pro-war prattle. Anything else is tantamount to heresy. The lights have gone out in American and only outside new services like BBC have the latitude to print and broadcast these truths and realities. Thanks BBC.
Dr W, PA, USA