Tony Blair has said Iraq's weapons of mass destruction "may never be found".
Blair: Not run out of steam
Mr Blair said he had "to accept we haven't found them and we may never find them" - but that did not mean Saddam Hussein had not been a threat.
He said the former Iraqi leader had been in breach of UN resolutions and his weapons may have been "removed, hidden or destroyed".
Mr Blair also said US security concerns had to be tackled before British detainees at Guantanamo Bay are freed.
The prime minister was being grilled by senior MPs in his twice-yearly appearance before the Commons liaison committee.
His comments come eight days before an inquiry reports on the pre-war intelligence about Iraq's weapons.
He said Saddam Hussein had previously had weapons of mass destruction and there was "very clear evidence" of his desire to develop and use them.
But he added: "I have to accept we haven't found them and we may never find them, We don't know what has happened to them.
"They could have been removed. They could have been hidden. They could have been destroyed."
In an interview broadcast later on BBC Radio 2, Mr Blair said there was no doubt Saddam Hussein had been a threat - even if the nature of the threat turned out to be different in some respects.
Liberal Democrat leader Charles Kennedy argued Mr Blair was diminishing his office by continuing to believe the weapons existed at the time he had tried to make the case for the war.
And Tory shadow foreign secretary Michael Ancram said Mr Blair owed the country an explanation after being so clear about Iraq's weapons before the conflict.
But US President George Bush told reporters later: "I know that Saddam Hussein was a threat."
The former Iraqi leader had possessed the intent and capability to produce weapons of mass destruction and had harboured terrorists, added Mr Bush.
Other highlights from Mr Blair's news conference included:
Mr Blair said extensive house building would go ahead in south-east England but insisted there was "no question of concreting over" the green belt
- The prime minister said police had been given new powers to deal with anti-social behaviour but he was willing to legislate again if there were still problems with using the law
- On energy, the door was still open to building more nuclear power plants if necessary, said Mr Blair.
Mr Blair mounted a robust defence of Britain's relationship with the US, saying other countries would "give their eye-teeth" to be in a similar position.
He said progress on security issues in Libya, Iran and North Korea would not have been possible "without Iraq".
But he added: "If I did not believe that the security of this country was enhanced by taking the action in Iraq, I would not have done it, irrespective of how many compliments the president [George Bush] paid me."
Guantanamo Bay is an 'anomaly', says Blair
The prime minister argued that Guantanamo Bay was "an anomaly that at some point has to be brought to an end".
He admitted the UK Government still did not have the "machinery" in place to ensure the remaining British prisoners there would not pose a security threat if they were released.
He confirmed that he had personally discussed the case of the four detainees with Mr Bush.
But he said he did not believe that the US was being obstructive in holding on to the detainees until the UK could give assurances that they would not be a security threat to it or any other country.
"I don't think the United States is being unreasonable. We need to make sure that there is proper security in place for these people," he told MPs.
Mr Blair confirmed a Cabinet committee on US-UK relations had been set up. He also said there would be no defence cuts in next week's spending review.
On race relations, Mr Blair warned concerns over international terrorism and the recent controversy over the stopping and searching of Asians were "new dimensions" that "we need to watch".
Mr Blair believes public opinion has shifted on smoking
"I know from my conversations with leaders of the Muslim community that they feel very strongly that if someone who calls themselves a Protestant goes on to the streets of Northern Ireland and murders a Catholic that doesn't reflect on the whole of the Protestant religion.
"Whereas they feel if you get Muslim extremists and terrorists then somehow this can be taken as stigmatising the entire community. I think we need to be sensitive to that."
Mr Blair also defended the concept of greater choice in public services, but said there had to be an expansion in "capacity" at the same time, otherwise the debate was "meaningless".
On health, he agreed about the importance of preventative measures and better education.
But he said he was also wary of creating a "nanny state" in relation to issues such as childhood obesity.
"In the end, I cannot tell someone how to live their lives," he told MPs.
However, the smoking debate had moved on, he added, allowing the government to contemplate legislation where a few years ago "people would have said what on earth are they doing".