The foreign secretary has announced the withdrawal of claims Iraq could launch weapons of mass destruction in 45 minutes.
He was speaking in the Commons for the first time since an official report confirmed that Saddam Hussein had no weapons of mass destruction at the time of the war,
But Jack Straw stood by the decision to launch the conflict, saying: "I still believe it was the correct decision."
In his statement to MPs, the foreign secretary announced that the claim Iraq could launch battlefield weapons of mass destruction within 45 minutes had now been withdrawn.
Straw confirmed the chief of the Secret Intelligence Service had written to parliament's intelligence and security committee "formally withdrawing" the claim.
Responding to that news, Sir Menzies Campbell of the Liberal Democrats said the "building blocks of the government's case for military action are crumbling before our eyes".
"The withdrawal of the 45 minute claim drives a horse and cart through government credibility," he added.
Straw also argued that the Iraq Survey Group's conclusions provided "chapter and verse" on the former dictator's intent to develop chemical and biological weapons and use them as he had done in the past.
He also said they showed Saddam was a current threat to the region and world despite the absence of an arsenal.
Quoting from the report, he said: "Iraq in 2003 probably had a capability to produce large quantities of sulphur mustard within three to six months."
Straw said the government had already acknowledged that key intelligence findings were wrong, but he did not use the word "sorry" as trade secretary Patricia Hewitt had done last week.
"As the prime minister did in his Labour Party conference speech I of course recognise that some of the evidence on which we based our decision was wrong," the foreign secretary said.
He paid tribute to the work of the intelligence services in uncovering Libya's weapons programme which he claimed, with less media interest, has led to the disarming of Colonel Gadaffi's regime.
But he argued that even with the flaws in intelligence the government made the correct decision to overthrow Saddam.
Straw said that the refusal to comply with UN inspections, even though there were no weapons, showed how difficult it was to trust the former regime.
"I do not accept, even in hindsight, that we were wrong to act," he said.
"It is still hard to comprehend the logic of Saddam's behaviour," he added.
"Even in the four days before the conflict Saddam refused our offer of just a few tests which would have shown Iraq had no weapons of mass destruction.
"In the circumstances giving Saddam the benefit of the doubt was not an option.
"It was the whole international community which on their own evidence, not our intelligence, concluded Saddam was a threat."
But Conservative spokesman Gary Streeter said Straw had not gone far enough.
"The British people must get a full apology and a full explanation," he said.
The foreign secretary also confirmed that his department would conduct an inquiry into what lessons can be learned from the case of British hostage Kenneth Bigley, who was executed in Iraq last week.
Pressed by Streeter as to whether there were any other steps the Foreign Office could have taken to resolve the crisis, Straw said: "I don't think there were but there will be a full internal review of what could have been done."