The minutes, published by The Sunday Times today, begins with the warning: “This record is extremely sensitive. No further copies should be made. The paper should be shown only to those with a genuine need to know.” It records a meeting in July 2002, attended by military and intelligence chiefs, at which Blair discussed military options having already committed himself to supporting President George Bush’s plans for ousting Saddam.

“If the political context were right, people would support regime change,” said Blair. He added that the key issues were “whether the military plan worked and whether we had the political strategy to give the military plan space to work”.

The political strategy proved to be arguing Iraq’s weapons of mass destruction (WMD) posed such a threat that military action had to be taken. However, at the July meeting Jack Straw, the foreign secretary, said the case for war was “thin” as “Saddam was not threatening his neighbours and his WMD capability was less than that of Libya, North Korea or Iran”.

Straw suggested they should “work up” an ultimatum about weapons inspectors that would “help with the legal justification”. Blair is recorded as saying that “it would make a big difference politically and legally if Saddam refused to allow in the UN inspectors”.

A separate secret briefing for the meeting said Britain and America had to “create” conditions to justify a war.

The papers, the second sensitive leak close to the election, appear to be an attempt by disaffected Whitehall insiders to attack Blair’s integrity. They are likely to fuel claims he misled the country on Iraq.

One reason for the secrecy is that the minutes record discussion of US plans for invasion; another is that at the time Blair had given no indication that plans were so advanced.

He had not revealed to MPs or the public that in April 2002 he had told Bush “the UK would support US military action to bring about regime change”, as recorded in the Foreign Office briefing paper. Both before and after the July meeting Blair insisted in public no decision had been made.

The July meeting was later mentioned by Lord Butler in his report on the use of intelligence on WMD as a “key stage” in the road to war; but its details have never been revealed until now.

The minutes show Goldsmith warned Blair eight months before war started on March 19, 2003 that finding legal justification would be “difficult”. The attorney-general only ruled unambiguously war was lawful a few days before the war started after Admiral Sir Michael Boyce, chief of the defence staff, demanded unequivocal written confirmation.

Boyce was never shown Goldsmith’s more equivocal advice to Blair of March 7, 2003, and says today ministers failed to give him protection from prosecution at the International Criminal Court. “I have always been troubled by the ICC,” he says, adding that if British servicemen are put on trial, ministers should be “brought into the frame as well”. Asked if that should include Blair and Goldsmith, he tells The Observer: “Too bloody right.”

Sir Menzies Campbell, Liberal Democrat foreign affairs spokesman, said the leaked minute showed Blair had “agreed to an illegal regime change with the Bush administration. It set out to create the justification for going to war. It was to be war by any means.”

Downing Street claimed the document contained “nothing new”.