Tony Blair has apologised for mistakes in pre-war intelligence on Iraq's weapons of mass destruction but has denied deliberately deceiving anyone.
Blair remains unrepentant over the decision to go to war
The prime minister came under further pressure from Tory leader Michael Howard who urged him to apologise for misrepresenting the intelligence.
Lib Dem leader Charles Kennedy accused him of waging an illegal war.
Mr Blair accused the Tories of "playing politics" and said the Lib Dems would have left Saddam Hussein in power.
The clashes follow the withdrawal of the claim Iraq could use the weapons at 45 minutes' notice.
At the first prime minister's questions since Parliament's conference break, Mr Howard said the intelligence services had said little was known about Iraq's chemical and biological programmes since 1998.
And yet Mr Blair had told the country it was "beyond doubt" that Saddam Hussein had continued to produce those weapons.
Mr Howard, who said he still backed the war, added: "How can the prime minister maintain he accurately reported the intelligence to the country. Why won't he say sorry for that."
In angry Commons exchanges the prime minister said he did not regret the war but said: "I have already apologised for any information that subsequently turns out to be wrong."
He demanded Mr Howard withdraw any suggestion there had been deception.
He countered that the Iraq Survey Group (ISG) had said the Iraqi leader had retained the intent, desire, capability and scientists for weapons programmes.
He said Mr Howard had four positions on the war - "three too many for anyone who seriously aspires to be prime minister".
Mr Blair taunted the Tory leader, saying: "What he is trying to do, having supported the war... is now capitalise on anti-war sentiment to try to give himself credibility".
He also said UN chief weapons inspector Hans Blix had backed a section of an early draft of the government's 2002 dossier on Iraq.
Mr Kennedy said regime change was now the only valid argument for the war as the 45-minute claim had been discredited and no weapons of mass destruction had been found.
"But the prime minister knows that regime change is contrary to international law... Isn't it the fact now, and shouldn't the prime minister accept, that he led us into an illegal war?"
That claim was rejected by Mr Blair, who said nobody had mentioned the 45-minute intelligence in the eve-of-war Commons debate.
The ISG had found that Iraq had been breaking UN resolutions, which made the case for war.
"It was a difficult choice," he said. "I took the choice, I stick by it."
Challenged by a Labour MP, Mr Blair said news on Wednesday of mass graves found in northern Iraq were proof of the appalling nature of Saddam Hussein's regime.
Although the prime minister seemed to successfully brush off criticism to cheers of his backbenchers, the Lib Dem foreign affairs spokesman Menzies Campbell said the issue was far from dead.
The bitter exchanges come the day after Foreign Secretary Jack Straw told MPs the 45-minute claim had been officially withdrawn by MI6.
The claim was at the heart of an unprecedented row between the BBC and the government which led to the death of David Kelly and the Hutton inquiry.
But Mr Straw insisted that "even with hindsight" knowing that Saddam had not had weapons of mass destruction, the war had been right.
The ISG report concluded last week that Iraq was free of weapons of mass destruction by the mid-1990s.
Later on Wednesday the Foreign Office issued new travel advice for Iraq, highlighting the increased risk of kidnapping.
It read: "There is a direct threat of kidnap to foreign nationals in northern Iraq and we believe that British nationals may be targeted."
It also warned that the number of attacks is likely to increase during Ramadan - 15 October to 14 November.