Downing Street's communications chief today angrily rejected what he called "very grave" charges that he induced the prime minister to mislead parliament and the British public over the premise for attacking Iraq.
Mr Alastair Campbell said an allegation he "sexed up" a dossier on Iraq's weapons to persuade reluctant legislators to back the war was blatantly false and he demanded an apology from the BBC, which aired the claim.
"It is a lie, it was a lie. It is a lie that is continually repeated and until we get an apology for it, I will keep making sure that parliament and the public know that it was a lie," Mr Campbell told the Foreign Affairs Committee, which is probing the use of intelligence before the war.
Mr Campbell is the closest that legislators will get to quizzing the prime minister about the row over Iraq's missing banned weapons.
The inability of US and British troops and experts to find Iraq's alleged arsenal of weapons of mass destruction has irked many in the ruling Labour Party and severely damaged the government's credibility.
Recent polls show that 34 per cent of the public is less likely to trust Mr Blair on other issues due to the weapons row.
His popularity has sunk and Labour's lead over the opposition Conservatives has plunged to its lowest level in over two years.
Mr Campbell was responsible for the compilation of a government document published in February that lifted sections of a student thesis.
Foreign secretary Mr Jack Straw told the same committee yesterday the dossier was "an embarrassment to the government".
Mr Campbell admitted today his team had made "a mistake", albeit an innocent one, with the compilation of the so-called 'dodgy dossier'.
But he dismissed the media coverage over the February dossier as "conspiracy nonsense".
Procedures were immediately put in place to ensure that similar mistakes were not made in the future, he said.
But Mr Campbell was outraged by the claim - made by an anonymous source speaking to the BBC - that he exaggerated evidence in an earlier September dossier on Iraq's weapons, thereby inducing Mr Blair to mislead parliament over the case for war.
He admitted to suggesting changes to the dossier but added: "It was, as it were, sexed down rather than up."
Speaking earlier to parliament, Mr Blair insisted that the intelligence presented in the weapons dossiers was accurate.