Labour yesterday suffered its worst day in the 2005 election campaign after Tony Blair finally succumbed to pressure to publish crucial legal advice on the Iraq war, but failed to stem the Conservative-led assault on his battered integrity.

Privately anxious cabinet ministers admitted that the renewed eruption of the Iraq issue - in the shape of the six-paragraph summary revealed on the Guardian Unlimited website on Wednesday night - may prove "a gift to the Tories" a week from polling day, not the "damp squib" of Mr Blair's prediction.

Ministers concede that the latest furore will probably push some demoralised Labour voters towards abstention.

Labour confirmed that Mr Blair and his advisers had decided yesterday to rush out a full version of the attorney general's interim legal advice, given 12 days before the war began, in the hope of proving it was consistent with his final advice that the war was legal.

Lord Goldsmith's legal opinion reveals the full extent of the attorney's concern about the risk of Britain being hauled before international courts which would even scrutinise allegations of war crimes by British troops.

It warns that British troops must use no more force than necessary to get Iraq to disarm. The attorney also makes it plain to Mr Blair that, in law, regime change could not be an objective of military action - a problem which did not concern the Bush administration.

His warnings to Mr Blair were not shown to the cabinet, which saw only Lord Goldsmith's later parliamentary answer, stripped of any of his earlier caveats.

As the Blair campaign tries to swing the election agenda back on to the economy today it will admit that concern over the Iraq-driven trust issue is such that it must warn wavering supporters that widescale switches to the Liberal Democrats could see Labour lose the election.

Labour will claim that if only one in 50 Labour voters in the 2001 election switches to the Lib Dems - or abstains - in the top 75 marginals, then Michael Howard will become prime minister. The decision to warn against a Lib Dem surge follows internal debate within Labour's high command over the past 48 hours.

The decision to publish the full 13 pages of Lord Goldsmith's advice, withheld from the public for two years, was announced by Mr Blair in a highly charged press conferences, during which he insisted he had not forced Lord Goldsmith, the attorney general, to change his advice on the legality of the war.

He claimed the leaked advice "was not a smoking gun, but a damp squib", insisting it showed that the attorney general did not change his mind from March 7, the date of his initial opinion, to March 17, the date he set out a legal basis for war to cabinet. The difference between March 7 and March 17 was that Lord Goldsmith "had to come to a definitive view".

At the press conference, Mr Blair insisted that once it became clear no UN resolution was possible, "I had to decide whether we back away, leave Saddam immeasurably stronger, or remove him. I took the decision to remove him. These decisions are tough and that is what leadership is about". He said he had worked to get "a second resolution, but had failed".

At one point he seemed to realise that he was no longer able to persuade anyone of the wisdom of his course.

"Whatever I say, I will never ever convince some people who have been opposed to this war. People can continue to frame this in terms of my integrity, but it was about a decision; I took it. I have to live with the consequences of it. I don't regret it. I cannot apologise. I tried very hard to find a middle way though, but I could not get it."

In a dark day for Labour campaign strategists, the one bright moment for No 10 was that Gordon Brown leapt to Mr Blair's defence and delivered a lucid and categoric defence of his old comrade.

Mr Brown also insisted that no propriety had been breached: "The attorney general had full discussions on numerous occasions with all members of the cabinet on all the issues relating to Iraq."

He added that the attorney general was "there to participate in the discussions and there to answer any questions we put. The decision was made in an honest, clear and principled way". Asked if he would have behaved in an identical way to the prime minister in the run-up to war he simply said "yes".

He went on: "I not only trust Tony Blair, but I respect him for the way he went about that decision involving all members of the cabinet."

The trade and industry secretary, Patricia Hewitt, also rallied to Mr Blair: "When the attorney general came to the cabinet meeting, he gave us his advice that conflict would be lawful. He spelt out that although a second resolution would have been preferable, it was not essential."

In the newly published advice of March 7, Lord Goldsmith warned Mr Blair that the government would be vulnerable to legal action were a second UN resolution not secured. "There are a number of ways in which the opponents of military action might seek to bring a legal case, internationally or domestically, against the UK, members of the government, or UK military personnel," he wrote.

"Given the strength of opposition to military action against Iraq, it would not be surprising if some attempts were made to get a case of some sort off the ground. We cannot be certain that they would not succeed."

These warnings deeply con cerned Lord Boyce, then chief of the defence staff. "I was looking for straightforward black and white legal advice," he told the Guardian yesterday, "not 'on the one hand, on the other'." He later got it - after the attorney sought advice on Iraq's weapons programme from Mr Blair.

Lord Goldsmith also warned in the clearest of terms that "regime change cannot be the objective of military action". He told Mr Blair: "This should be borne in mind in considering the list of military targets and in making public statements about any campaign."

Regime change was a clear objective for the Bush administration. Mr Blair had to be careful what he said in public.

Both the two main opposition parties seized on the attorney general's advice.

Michael Howard said the "devastating" leak of the attor ney general's advice showed that the prime minister had deceived parliament.

"If you cannot trust Mr Blair on the decision to take the country to war, the most important decision that any prime minister can take, how can you trust Mr Blair on anything else, ever again?"

Charles Kennedy yesterday urged the prime minister to disclose all remaining material relating to the attorney general's legal advice on Iraq. The Liberal Democrat leader said the public needed to know why the advice had changed over the 10 intervening days. "What minutes, what meetings, what discussions, what decisions - who was involved?" Mr Kennedy asked.

Opinion from senior lawyers was divided last night about the implications of the attorney general's advice. Sir Franklin Berman, a former Foreign Office legal adviser, praised Lord Goldsmith for a "very impressive piece of advice". But Sir Adam Roberts, professor of international relations at Oxford, said: "The fact that some cabinet ministers had to wait until now to see this fuller advice suggests that Downing Street was as unprofessional in handling the legal issues as it was the intelligence issues. The public is entitled to a proper apology."