The reports, from papers leaked to the Daily Telegraph, claim Mr Straw predicted a post-war Iraq would cause major problems, telling Mr Blair that no-one had a clear idea of what would happen.
And he questioned the US claims that an invasion would eliminate the threat of weapons of mass destruction, saying "no one has satisfactorily answered how there can be any certainty that the replacement regime will be any better."
In a "Secret UK Eyes Only" paper, senior ministerial advisors are said to have warned that "the greater investment of western forces, the greater our control over Iraq’s future, but the greater the cost and the longer we would need to stay."
The only certain way to create a new government, they said, would involve "nation-building" over many years.
The papers also warn that any "Sunni strongman" put in power to replace Saddam could face a series of military coups, which would leave the country as unstable as it was before the war.
Another leaked confidential memo by a senior Foreign Office official written in March 2002 questioned the idea that regime change in Iraq was the real reason for war.
"Military operations need clear and compelling military objectives. For Iraq, ‘regime change’ does not stack up. It sounds like a grudge match between Bush and Saddam (Hussein)."
The revelations were described as "devastating" by Liberal Democrat foreign affairs spokesman Sir Menzies Campbell.
"If these documents are accurate they provide a devastating insight into the political run up to war in Iraq," he said.
The Independent has more details, including actual quotes:
An extraordinary cache of leaked documents this weekend forced Mr Blair to deny that the planning for post-invasion Iraq had been inadequate. Amid a deteriorating security situation in the country, Mr Blair blamed terrorists for trying to stop the creation of a stable and democratic Iraq. But the leak also lays bare the gulf between what Mr Blair and his aides said in public about Iraq and their private discussions with the White House.
On the eve of Mr Blair's visit to Mr Bush's ranch in Crawford, Texas, in March 2002, for example, Mr Blair's official spokesman dismissed suggestions that he was going for a "council of war". The leaks show, however, that Sir David Manning, his foreign policy adviser, had already assured Condoleezza Rice, Mr Bush's National Security Adviser, that Mr Blair was fully signed up to toppling Saddam.
In a memo to the Prime Minister dated 14 March 2002, Sir David says he told Ms Rice "you would not budge in your support for regime change". Four days later, Downing Street received a despatch from the then British ambassador to the US, Sir Christopher Meyer, detailing how he repeated the commitment to Paul Wolfowitz, the US Deputy Defence Secretary. The ambassador added that Mr Blair would need a "cover" for any military action. "I then went through the need to wrongfoot Saddam on the inspectors and the UN Security Council resolutions."
Taken with the Joint Intelligence Committee's warning at the same time that intelligence on Iraqi WMD was "sporadic and patchy", the account appears to prove the charge that Mr Blair's use of the issue was purely tactical.
The question of Iraqi WMD comes back sharply into focus next week with the final report of the Iraq Survey Group. An early draft of the report said to be circulating in Washington concludes that the regime was working on only small amounts of biological and chemical agents probably for use in assassinations.
Equally embarrassing are leaked documents showing that Jack Straw, the Foreign Secretary, repeatedly warned Mr Blair that the US was failing to answer the "big questions" about what would happen after Saddam was overthrown.
On 25 March 2002, in a private note to the Prime Minister, Mr Straw detailed the difficulties he faced in winning over his own MPs and the public. "What will this action achieve?" he asked. "There seems a larger hole in this than anything." The US, he said, had failed to answer the question of how "there can be any certainty that the replacement regime will be any better".
A recent briefing paper marked "Secret UK eyes only", and also leaked this weekend, warned that Iraq could "revert to type", with "coup succeeding coup" if Saddam was replaced with another Sunni "strongman".
Australians are entitled to know if the Man of Steel also leapt on board the Bush drive for war a year before publicly admitting it. Blair denies there was no planning. If this is planning, what would an unplanned occupation look like?