Tony Blair agreed to commit British troops to battle in Iraq in the full knowledge that Washington had failed to make adequate preparations for the postwar reconstruction of the country.

In a devastating account of the chaotic preparations for the war, which comes as Blair enters his final full week in Downing Street, key No 10 aides and friends of Blair have revealed the Prime Minister repeatedly and unsuccessfully raised his concerns with the White House.

He also agreed to commit troops to the conflict even though President George Bush had personally said Britain could help 'some other way'.

The disclosures, in a two-part Channel 4 documentary about Blair's decade in Downing Street, will raise questions about Blair's public assurances at the time of the war in 2003 that he was satisfied with the post-war planning. In one of the most significant interviews in the programme, Peter Mandelson says that the Prime Minister knew the preparations were inadequate but said he was powerless to do more.

'Obviously more attention should have been paid to what happened after, to the planning and what we would do once Saddam had been toppled,' Mandelson tells The Observer's chief political commentator, Andrew Rawnsley, who presents the documentary.

'But I remember him saying at the time: "Look, you know, I can't do everything. That's chiefly America's responsibility, not ours."' Mandelson then criticises his friend: 'Well, I'm afraid that, as we now see, wasn't good enough.'.

Opponents of the war, who have long claimed that the Pentagon planned a short, sharp offensive to overthrow Saddam Hussein with little thought of the consequences, claimed last night that the programme vindicated their criticisms. Sir Menzies Campbell, the Liberal Democrat leader, told The Observer: 'These frank admissions that the Prime Minister was aware of the inadequacies of the preparations for post-conflict Iraq are a devastating indictment.'

Blair's most senior foreign affairs adviser at the time of the war makes clear that Blair was 'exercised' on the exact issue raised by the war's opponents. Sir David Manning, now Britain's ambassador to Washington, says: 'It's hard to know exactly what happened over the post-war planning. I can only say that I remember the PM raising this many months before the war began. He was very exercised about it.'

Manning reveals that Blair was so concerned that he sent him to Washington in March 2002, a full year before the invasion. Manning recalls: 'The difficulties the Prime Minister had in mind were particularly, how difficult was this operation going to be? If they did decide to intervene, what would it be like on the ground? How would you do it? What would the reaction be if you did it, what would happen on the morning after?

'All these issues needed to be thrashed out. It wasn't to say that they weren't thinking about them, but I didn't see the evidence at that stage that these things had been thoroughly rehearsed and thoroughly thought through.'

On his return to London, Manning wrote a highly-critical secret memo to Blair. 'I think there is a real risk that the [Bush] administration underestimates the difficulties,' it said. 'They may agree that failure isn't an option, but this does not mean that they will avoid it.'

Within a year Britain lost any hope of a proper reconstruction in Iraq when post-war planning was handed to the Pentagon at the beginning of 2003.

Sir Jeremy Greenstock, Britain's envoy to the postwar administration in Baghdad, confirms that Blair was in despair. 'There were moments of throwing his hands in the air: "What can we do?" He was tearing his hair over some of the deficiencies.' The failure to prepare meant that Iraq quickly fell apart. Greenstock adds: 'I just felt it was slipping away from us really, from the beginning. There was no security force controlling the streets. There was no police force to speak of.'

The revelation that Blair was 'exercised' in private will raise questions about his public assurances. The former Labour leader, Neil Kinnock, told the programme he was given a personal assurance by Blair that he was satisfied by the preparations. 'I said to Tony, are you certain?' Kinnock told the programme. 'And when he said: "I'm sure," that was a good enough reassurance.'

Condoleezza Rice, then Bush's national security adviser, confirms that the President offered Blair a way out. Bush told Blair: 'Perhaps there's some other way that Britain can be involved.' Blair replied: 'No, I'm with you.'

· The Rise and Fall of Tony Blair will be screened on Channel 4 next Saturday.