Senior figures in the intelligence community and across Whitehall briefed the former international development secretary Clare Short that Tony Blair had made a secret agreement last summer with George Bush to invade Iraq in February or March, she claimed yesterday.

In damning evidence to the foreign affairs select committee, Ms Short refused to identify the three figures, but she cited their authority for making her claim that Mr Blair had actively deceived the cabinet and the country in persuading them of the need to go to war.

Ms Short told the first day of the committee's inquiry into the events leading up to the Iraq conflict that Mr Blair had "used a series of half-truths, exaggerations, reassurances that were not the case to get us into conflict by the spring".

She claimed Mr Blair told President Bush that "we will be with you" without laying down conditions to temper US ambitions.

She went on: "I believe that the prime minister must have concluded that it was honourable and desirable to back the US in going for military action in Iraq and therefore it was honourable for him to persuade us through various ruses and ways to get us there - so for him I think it was an honourable deception."