Conservative leader Michael Howard says Mr Blair lied over the war. Liberal Democrat leader Charles Kennedy says that "there is no doubt that we were misled" with regard to "the threat that we were under as a country" and "the true aims of government policy" and has called for an independent inquiry
On 24 September 2002 the UK government published an intelligence dossier outlining its concerns over Iraq's weapons of mass destruction, including the claim that Iraq could deploy chemical and biological weapons within 45 minutes.
In the foreword, Mr Blair said that "this issue was a current and serious threat to the UK national interest".
On 18 March 2003, just before the UK went to war with Iraq, Mr Blair told the House of Commons that it was "palpably absurd" to accept that Saddam Hussein "contrary to all history, contrary to all intelligence" had "decided unilaterally to destroy these weapons".
Since the war extensive searches by the US-led Iraq Survey Group after the war failed to uncover any nuclear, chemical or biological weapons.
The Oxford dictionary defines a lie as a "statement the speaker knows to be untrue".
The January 2004 Hutton report, into the circumstances of the death of government scientist Dr David Kelly, said the allegation that the September 2002 dossier was embellished with items of intelligence known or believed to be false or unreliable was "unfounded".
But it accepted that the prime minister's desire to have as compelling a dossier as possible may have subconsciously influenced the Joint Intelligence Committee to make the language of the dossier "stronger than they would otherwise have done".
The Butler report, published in July 2004, was set up to investigate the accuracy of intelligence on Iraqi weapons of mass destruction up to March 2003.
It concluded that much of the evidence in the dossier was "unreliable" or "very thin" and "it was a serious weakness" that "warnings on the limitations of the intelligence underlying its judgements were not made sufficiently clear".
It added: "Language in the dossier and used by the prime minister may have left readers with the impression that there was fuller and firmer intelligence than was the case."
But the report said it had no evidence of deliberate distortion of the intelligence.
With hindsight, everyone agrees that much of the intelligence that the UK (and US) government published to justify their case for war against Iraq was unreliable.
Mr Howard believes Mr Blair lied. He told Breakfast with Frost: "The intelligence that he had, as we know from the Butler report... was limited sporadic and patchy. When Mr Blair came to report that to the country, he said he had intelligence that was extensive, detailed and authoritative. Maybe you can reconcile those two different sets of words. I can't. I think that portraying the intelligence in that way was untrue."
The Liberal Democrats do not accuse Mr Blair of lying, but they say the UK was taken to war on a "false prospectus". Pressed on whether Mr Blair had lied, Lib Dem foreign affairs spokesman Sir Menzies Campbell said he could not say that because: "Only he (Tony Blair) knows if he was telling the truth when addressing the House of Commons."
Mr Blair denies lying or misrepresenting the intelligence on Iraq's weapons. He has acknowledged (at September's Labour Party conference) that "the evidence about Saddam having actual biological and chemical weapons, as opposed to the capability to develop them, has turned out to be wrong. I acknowledge and accept that. I simply point out that it was agreed by the whole international community."
He says that, even now knowing that the intelligence was wrong, he cannot apologise for taking the tough decision to remove Saddam Hussein, and says he believes the world is a better place without him. He has consistently stressed that he respects the views of opponents of the war.
Ultimately, as he has said on a number of occasions, it is a question of his judgement rather than his character - and voters will have the chance to deliver their verdict on polling day.