proved to be vital for providing a case for war that side-stepped the need for a second UN resolution: the so-called 'revival argument'. This relied on linking three UN resolutions: 678, which authorised the use of force in removing Iraqi forces from Kuwait in 1990; 687, which set the ceasefire conditions at the end of the war in 1991, including the dismantling of weapons of mass destruction; and 1441, which threatened 'serious consequences' if those conditions were breached.

In his 7 March legal advice, Lord Goldsmith makes it clear that some British law officers believed that it was up to the Security Council, not individual states, to decide if Iraq was in breach of its obligations.

But Goldsmith discloses that he had fully taken on board the arguments made to him during his visit to Washington: 'The US have a rather different view: they maintain that the fact of whether Iraq is in breach is a matter of objective fact, which may therefore be assessed by individual member states. I am not aware of any other state which supports this view.'

The advice clarifies a second vital point: that the American legal advisers who drew up 1441 were convinced that it contained, in itself, the authorisation to use force against Saddam if he could be shown to have failed to disarm.

Goldsmith refers specifically to his meetings with the neo-cons and the effect the arguments that Taft and others had on him: 'I was impressed by the strength and sincerity of the views of the US administration which I heard in Washington on this point.'

Taft remains adamant that 1441 gave the US and Britain a legitimate trigger for the use of force. 'We were drafting the resolution having in mind that we might not get another one and we wanted, in the event of non-compliance [by Iraq], to allow our policy-makers to be in a position to do what they needed to do,' he said. 'There was an enormous fight. A draft resolution was tabled stating that the Security Council would have to take further action and this was not accepted.'

Taft is convinced that Goldsmith's final advice to Blair was correct under international law. 'I am still right there. The use of force was entirely lawful and authorised by the Security Council.'

THhe anger of defence officials

By the end of February 2003, more than 50,000 British troops had been sent to northern Kuwait preparing for an assault on Iraq if the order was given.

While the military strategy was going according to plan, back in the Ministry of Defence HQ, Admiral Sir Michael Boyce, the then Chief of Defence Staff, was growing increasingly impatient. Aware of the political debate raging on the legality of the war, he told the Prime Minister he would need 'black and white' legal cover before he ordered the troops in.

As The Observer revealed last March, it was Boyce's fears that soldiers might face prosecution for war crimes before the International Criminal Court that led to his demands for an 'unequivocal' two-line note from the Attorney General giving him the green light.

Boyce's clear demand for this unambiguous statement was transmitted to Goldsmith through the Prime Minister. Boyce's stance has emerged as a key factor in pushing Goldsmith to reconsider his final advice, which was presented to the Cabinet on 17 March and which removed many of his caveats in the 7 March document.

In an interview with The Observer, Boyce said: 'My requirement for legal "top cover" didn't start on 7 March; it started from the time we were getting troops in the area at the back end of February. It was obvious already there was quite a heavy debate going on that lent an air of uncertainty to our troops.

'My concern was that the troops should feel absolutely confident that what they were doing was absolutely black and white legally. So, well before 7 March, I made it clear to the Prime Minister that before we went in we would require legal top cover.'

Boyce said he never saw Goldsmith's 13-page full legal opinion of 7 March. 'I didn't see it ... it was not copied to me. My concerns weren't aroused or sparked by that, but by general concern.

'What the Attorney General didn't say was that it was illegal. It had caveats in it ... and some time between producing that piece of work on 7 March and his ultimate instruction or advice to Cabinet and Parliament and to me ... whatever things that were required to remove the caveats or to consider them to be not so important, I am not privy to. At the end of the day, he provided me with the unequivocal black-and-white advice that it was legal for us to go in.'

Yet in a remarkable admission, when asked if he believed he had finally received the necessary legal cover to protect him from a prosecution before the ICC, Boyce replies: ' No - I think I have done as best I can do. I have always been troubled by the ICC. Although I was reassured at the time when it [the decision to sign up to the ICC] was going through Whitehall about five years ago, I was patted on the head and told, don't worry: on the day it will be fine. I don't have 100 percent confidence in that.'

Goldsmith himself warned explicitly of the danger in his 7 March advice that such an attempted prosecution was a possibility. He stated: 'We cannot absolutely rule out that some state strongly opposed to military action might try to bring such a case. It is also possible that CND may try to bring further action to stop military action in the domestic courts. Two further possibilities are an attempted prosecution for murder on the grounds that the military action is unlawful and an attempted prosecution for the crime of aggression.'

Boyce reveals for the first time that part of his motive for insisting on legal assurance was to ensure that, should he or his soldiers face prosecution for the war, then his political masters would also be called to account. He said: 'My concerns were that, if my soldiers went to jail and I did, some other people would go with me - and that's what I had. I had a perfectly unambiguous black-and-white statement saying it would be legal to operate if we had to.'

Asked whether he meant by 'other people' Lord Goldsmith and Tony Blair he replied: 'Too bloody right!'

'I wanted to make sure we had this anchor that has been signed by the government law officer to show that at least we were operating under legal advice. It may not stop us from being charged, but, by God, it would make sure other people were brought in the frame as well.'

Blair's dark week

For nearly five million Britons, polling day came early this year. The ballot boxes may not open until 5 May, but the postal votes on which many of Labour's most marginal seats will be decided began dropping on to doormats last weekend. And somebody was ready for them.

Beneath the headline 'The proof Blair was told war could be illegal', last week's Mail on Sunday detailed the long-suspected, but never before confirmed, reservations and caveats of the Attorney General. The paper did not have the document itself, but had been read out enough authentic detail for Goldsmith to issue a statement admitting it was genuine.The Government's best kept secret was out - just as postal voters' pens were poised over their ballot papers.

Straw was hauled on to the Today programme on Monday for what one colleague called a 'painful' performance defending the legality of the war. For Straw personally, already struggling with an angry Muslim vote in his Blackburn constituency, it was the worst possible time to be thrust into an argument over Iraq.

The week stumbled from bad to worse. On Tuesday, it was announced that Brian Sedgemore, a veteran Labour MP, was defecting to the Lib Dems. The result was Tuesday's damning headlines, with Sedgemore accusing Blair of 'stomach-turning lies' over the war. It was the final encouragement Tory leader Michael Howard's team needed: the next morning, they launched their own poster accusing Blair of lying to go to war, and asking what else he might lie about to get re-elected.

Blair was in the middle of an interview for Sky on Wednesday, insisting he did not tell lies, when Channel 4 News began reporting an altogether more interesting story. It was another leak of the war advice: crucially, this time it and the Guardian had a copy of Goldsmith's summary - albeit handwritten to disguise the source. The crucial part of the document the government had fought for two years to conceal was now posted on a newspaper website for all to see.

Labour ministers are unanimous in blaming the Tories for having somehow orchestrated the first leak, even though Tory aides insist that is 'pure paranoia'.

It was the second hit that really lit the fuse. The Trade and Industry Secretary, Patricia Hewitt, watching the news at home, shot to her computer to download the document she had never been shown - despite being in the cabinet. By the following morning, John Prescott, the Deputy Prime Minister, was being chased from a press conference by a reporter brandishing a copy of the advice and shouting: 'Do you want this? You haven't read it.'

The farce overshadowed the unfortunate fact that the full 13-page legal advice did not prove Howard's central charge. Admittedly, Goldsmith's conclusion, in paragraph 28, that 'a reasonable case can be made' for war without a second UN resolution was well short of a ringing endorsement, but neither did he say the war was unlawful. Blair had not lied when he said the Attorney General had ruled the war legal in the end - although he had omitted to mention that the definitive verdict was certainly not there as late as 7 March.

By the time Blair took the podium in the basement of the financial wire service Bloombergs' City office on Thursday morning, ostensibly to launch the party's business manifesto, it was clear there would be few questions about corporation tax. Blair angrily dismissed the leaked document as a 'damp squib': asked why, in that case, it shouldn't be published in full, he startled the audience by announcing it might as well since 'you have probably got it all anyway'.

But the crucial intervention came not from Blair, but from the man beside him. Asked if he, too, would have gone to war in Blair's position, Gordon Brown responded with a succinct: 'Yes.' The spontaneous applause came not from the bemused audience of businessmen, but from relieved Labour aides. Brown had passed up a golden chance to make personal capital out of the war - even though he could hardly have done otherwise in public. Hewitt, sharing the platform, could not resist exclaiming: 'Well done.'

Help also came from an unexpected source. At his morning press conference, Howard was asked whether - given the same legal advice - he, too, would have invaded, he confirmed he would. The logic of his position was crumbling.

On Thursday night, as the three leaders submitted to the David Dimbleby treatment on Question Time , Howard went still further, confirming he would have gone to war knowing there was no WMD. Saddam was still a threat, he said, and he favoured 'regime change-plus' - words anti-war voters love to hate.

When Howard asked the Question Time audience how many people thought Blair had told the truth on Iraq, few hands shot up. And yet his decision to 'go negative' on Blair's character now looks like an own goal: a Populus poll yesterday showed that almost half of voters were less likely to vote Tory as a result. Whatever people think of Blair, they apparently don't want to hear it from Howard.

The danger for Labour, however, comes not just from Howard. Charles Kennedy's stance on Iraq already puts him in pole position for anti-war votes: his mission now is to vacuum up floating voters, particularly women, who hate not the war, but the playground shrillness of debate about it.

His trick of hovering statesmanlike over the fray - Kennedy has criticised both Howard's choice of language and Blair's angry dismissal of the legal advice as a 'damp squib' - may be trickier to maintain given his own party's latest election broadcast, which caricatures the Prime Minister as the boy who cried wolf over WMD. But in about half a dozen seats, such as Islington South or Hornsey and Wood Green in London, Labour strategists admit that the furore over the legal advice could be enough to swing them Kennedy's way.

No wonder one minister, in a seat way down the Tories' hit list, is now writing two speeches for election night, one for winning and one for losing: 'It's undoubtedly close. Iraq, Blair and council tax are the three issues that keep coming up, and the answers I'm giving are not the sort that people want to hear.'

Others on the front line, however, insist that minds are already made up about the war, legal advice or no legal advice. 'If people are going to [vote against me] on the war, they tend to have been very definite from the start,' says one experienced Labour MP fighting a stiff Lib Dem challenge. 'They have had a couple of years to think about it, after all.'

There is, of course, an incentive for Labour to exaggerate the danger: if the election looks 'in the bag', as Alastair Campbell did not quite say last week, reluctant supporters will not bother to vote. A growing number of ministers are now arguing for an extended diet of humble pie, even if Labour is returned with a healthy majority. There must, they argue, be no triumphalism, and not just over Iraq: too many voters are angry and disillusioned about issues ranging from public services to immigration.

'If we get back with a reduced majority, we cannot have a scintilla of arrogance: he's got to show he's clocked it,' says one senior minister. Sedgemore's claims this weekend, in a GMTV interview, that 'hundreds' of MPs are poised to turn on the leader after the election are exaggerated, but his own mini-revolt is only one of several being planned on the left once the election is over.

There are threats of a stalking horse candidate running against Blair if he refuses to resign forthwith: Bob Marshall Andrews MP, the maverick left-winger, has publicly spoken of 'serious movements' to change the leader. A string of left-wing MPs, dutifully silent during the election, are planning speeches in the first two weeks of May, calling for a radical change of direction for the party.

Nonetheless, the threat of a revolt has been lessened by Brown and Blair's campaign rapprochement - and, in a shrunken parliamentary party, it may be uphill work to find the 82 MPs' signatures necessary for a coup. A majority of anything over 50, and Blair is probably safe: winning fewer than 209 seats, and Howard is not. But even a halved majority would rattle nerves.

One Blairite minister gloomily predicts 'a bloodbath' after polling day, as recriminations are traded between Brown and Blair camps for lost seats: 'This is the calm before the storm. I may be wrong, but I'm worried it will be the two sides back at it again.'

The young woman who stepped into Tony Blair's path as he finished his whirlwind tour of a nursing home in Risley, Northamptonshire, on Friday afternoon was nervous but emphatic. 'I just wanted to say, don't rise to the bait,' she told him, adding that she didn't want to get political but was upset by the tone of the Tory campaign.

Blair must now hope that Iraq, for all the damage it has done in the past week, is now finished business, at least for the rest of the campaign.

Today's revelations of the American meetings, the anger of Boyce and the faltering Labour lead in the polls will ensure that those around Blair continue to bite their nails as the last 96 hours of the campaign hove into view. Four days to go - four days during which Britain's longest serving Labour Prime Minister will wonder, as he races from town to town, speech to speech, interview to interview, if the result on 6 May is really in the bag.