Former Cabinet ministers Robin Cook and Clare Short revealed yesterday they were told by MI6 in the run-up to the war with Iraq that Saddam Hussein's weapons of mass destruction did not pose any immediate threat.

At the opening session of the Commons Foreign Affairs Committee inquiry into the use of intelligence in the Iraq conflict, both ex-ministers said they believed the threat from Iraqi weapons had been exaggerated.

Ms Short repeated her accusation that Tony Blair had "pre-committed" Britain to war, even though he had claimed to be working for a resolution to the crisis through the United Nations.

She said that she believed Mr Blair must have convinced himself that he was engaged in an "honourable deception" because he thought that it was right for Britain to back the US over military action.

Mr Cook stopped short of accusing Downing Street of deliberate deception, but he said there had been a "burning fixation" with Iraqi weapons and that intelligence had been used selectively to support a policy which had already been decided.

However, the most damaging evidence for Downing Street was the disclosure by the two MPs that intelligence briefings by MI6 before the war had made clear that there was no imminent threat from Iraqi chemical or biological weapons.

Mr Cook stopped receiving regular intelligence briefings after he left the Foreign Office in 2001 to become Leader of the Commons.

But he, along with the rest of the Cabinet, had been briefed individually by MI6 - more properly known as the Secret Intelligence Service (SIS) - in the run-up to war.

His resignation speech - when he said Iraq probably had no weapons of mass destruction in terms of "a credible device capable of being delivered against a strategic city target" - had reflected "almost word for word" that briefing.

Mr Cook said that even if some chemical agents or biological toxins were now discovered, it would not justify Mr Blair's claim before the war that Iraq represented a "serious and current threat".

Ms Short described the Government's claims about Iraqi weapons before the war as a "series of half truths and exaggerations"