In an exclusive interview with the Guardian, Geoff Hoon reveals that Britain disagreed with the US administration over two key decisions in May 2003, two months after the invasion - to disband Iraq's army and "de-Ba'athify" its civil service. Mr Hoon also said he and other senior ministers completely underestimated the role and influence of the vice-president, Dick Cheney.

"Sometimes ... Tony had made his point with the president, and I'd made my point with Don [Rumsfeld] and Jack [Straw] had made his point with Colin [Powell] and the decision actually came out of a completely different place. And you think: what did we miss? I think we missed Cheney."

Giving the most frank assessment of the postwar planning, Mr Hoon, admits that "we didn't plan for the right sort of aftermath".

"Maybe we were too optimistic about the idea of the streets being lined with cheering people. Although I have reconciled it in my own mind, we perhaps didn't do enough to see it through the Sunni perspective. Perhaps we should have done more to understand their position."

He said history would have to decide whether the coalition should have anticipated the Sunni-Shia violence. "Given what we know now, I suppose the answer is that we should, but we did not know that at the time."

Of the summary dismissal of Iraq's 350,000-strong army and police forces, Mr Hoon said the Americans were uncompomising: "We certainly argued against [the US]. I recall having discussions with Donald Rumsfeld, but I recognised that it was one of those judgment calls. I would have called it the other way. His argument was that the Iraqi army was so heavily politicised that we couldn't be sure that we would not retain within it large elements of Saddam's people."

Mr Hoon, now minister for Europe, accepted that the sacking of so many Iraqis in possession of weapons and military training had been catastrophic, allowing "Saddam's people to link up with al-Qaida and to link up ultimately with Sunni insurgents" in fomenting suicide attacks and sectarian violence.

He added that the military advance on Baghdad had been so rapid that "Saddam's people had been left in its wake, who were not defeated. Partly because they were not wearing uniforms and they didn't line up and fight ... classically what was happening was that the advance went forward and the insurgents were coming in at the back ... simply picking off, at the sides, the people they could attack.

"And that created this situation in which it was possible for Saddam's people to link up with al-Qaida and to link up ultimately with Sunni insurgents, and ... this has grown".

The dismantling of several ministries and removal from office of all state employees with Ba'ath party membership was also an error, Mr Hoon says.

The decision is widely seen to have paralysed the country's infrastructure. "I think we probably saw it in a different way [to the US]. I think we felt that a lot of the Ba'ath people were first and foremost local government people, and first and foremost civil servants - they weren't fanatical supporters of Saddam."

Describing the task of dealing with the US administration as a "multi-dimensional jigsaw puzzle," Mr Hoon accepted that Britain had greatly underestimated the influence of the neo-con vice-president Mr Cheney and had lacked a comparable figure able to engage him regularly over the war. And he admitted that as Mr Powell became more marginalised by the White House and Mr Rumsfeld's Pentagon, Britain's coordination of its position through his state department left its influence greatly diminished.

Mr Hoon also expressed regret over the government's claim in the run-up to war that Saddam Hussein possessed weapons of mass destruction, which, he now accepts, turned out to be false. He said he had "gradually come to the acceptance" the weapons did not exist. But he insisted the government had acted in good faith.

He still does not understand why the intelligence proved to be false. "I've been present at a number of meetings where the intelligence community was fixed, and looked in the eye and asked are you absolutely sure about this? And the answer came back 'Yes, absolutely sure'."

Mr Hoon added: "I saw intelligence from the first time I came into office, in May 1999 - week in, week out - that said Saddam had weapons of mass destruction ... I have real difficulty in understanding why it was, over such a long period of time, we were told this and, moreover, why we acted upon it."

Mr Hoon accepted that the public hostility towards Mr Blair is considerable. "No one's interested in subtleties of judgment or what was the case at the time. I think, especially when British soldiers are being killed, that the public have got to be pretty confident as to why. I think they're not any longer confident, and want us out of Iraq. That's why Tony gets the blame."

"Whatever else I did, even if say people say it was catastrophically wrong, I wouldn't agree with it, but I could live with it. But I can't live with the idea that I was telling lies, because I wasn't."

On the question of an apology, he says: "That's the whole thing about apologising, and saying we were wrong. - it's quite hard. You can say "it did not turn out as we expected" and "we made some bad calls", but at the end of the day I defy anyone to to go through what we went through and come to a different conclusion".