Tony Blair has told the Hutton inquiry he would have resigned if the claims about the government "sexing up" its Iraq dossier had been true.
Tony Blair returns to Downing Street
The prime minister was giving evidence to the inquiry into the death of government scientist Dr David Kelly, the source for the BBC report about intelligence in last September's dossier being exaggerated to make the case for war.
That report threatened his credibility as prime minister and, if true, "would have merited my resignation", said Mr Blair.
He has also told the inquiry he took responsibility for the media strategy which led to Dr Kelly being publicly named as the suspected source of the BBC Today programme story.
In other developments:
During his two hours and twenty minutes of evidence Mr Blair said the dossier was not to be used "as the immediate reason for going to conflict".
- Conservative leader Iain Duncan Smith accused the prime minister of "underhand" and "shameful" treatment of Dr Kelly
- Liberal Democrat leader Charles Kennedy said trust in the prime minister had been undermined and he questioned whether, "knowing what we know now", MPs would vote for war with Iraq
- BBC chairman of governors Gavyn Davies defended the corporation's right to broadcast the claim that the Iraq weapons dossier was "sexed up"
- Mr Blair telephoned Mr Davies to tell him an official had come forward who they thought was the BBC's source, the day before a press release announcing it was issued
Mr Blair said he had not been aware of any unhappiness about the dossier among intelligence officers.
When he had heard BBC journalist Andrew Gilligan's report, he had asked for the claims to be checked with John Scarlett, the chairman of the Joint Intelligence Committee.
He described a "raging storm" which erupted in the wake of the BBC story and argued the inclusion of Mr Campbell's name later in the story meant it was "no longer a small item".
Mr Blair said that although Mr Scarlett was in charge of the dossier's drafting, he had had "no doubt" that Number 10 press chief Alastair Campbell would help with the presentation of the document.
Once the row blew up he thought Parliament's Intelligence and Security Committee, rather than the Commons Foreign Affairs Committee, would need to investigate it. In the end both did.
Inquiry counsel James Dingemans asked if Mr Campbell's live appearance on Channel 4 had helped escalate the row between Downing Street and the BBC.
Mr Blair said the dispute was there already, but continued: "For us... the dispute was in a sense not what was important. What was important was to correct the story."
Mr Blair admitted he was in a "quandary" about what to tell the two committees of MPs after Dr Kelly admitted to his bosses at the Ministry of Defence that he had met Mr Gilligan.
But he said: "We handled this by the book, in the sense of with the advice of senior civil servants.
"Not as I say in order to pass responsibility to them but in order to make sure that this was not as it were the politicians driving the system, but us taking a consensus view about what was the right way to proceed."
It was decided to tell MPs' committees the suspected source had come forward - as it would have been improper to keep it from them - before making the news public without naming Dr Kelly.
The feeling was: "Look, we are best simply to be open about this. We know this information."
The prime minister said he took full responsibility for those decisions.
He had not seen the "question and answer" sheet which told Ministry of Defence press officers they could confirm Dr Kelly's name if it was put to them by journalists.
But he said: "I think the basic view would have been not to offer the name but on the other hand not to mislead people."
He continued: "The trouble was it was fairly obvious the name was going to come out. The most that you were doing with the public statement was a getting a short breathing space."
Nothing in the discussions had suggested the scientist was "anything other than someone of a certain robustness who was used to dealing with the interchange between politics and the media".
Mr Blair said he did not know why his official spokesman had given further details about Dr Kelly at a lobby briefing.
Earlier, Mr Blair said it was "absolutely wrong" for BBC witnesses to have suggested the reason he had not mentioned the 45 minute claim again was because the government had doubts about it.
The prime minister was greeted by a noisy protest from about 100 anti-war protesters as he arrived to become only the second ever British prime minister to go before a judicial inquiry.
He was followed into the witness box by the BBC chairman Gavyn Davies, who accused Mr Campbell of escalating the row over the Today story in an "unprecedented attack" on the corporation.