Downing Street has published the full advice it received on the legality of the Iraq war, after fresh media leaks.
The issue of Iraq is firmly back on the agenda
It shows that the attorney general told Tony Blair on 7 March 2003 a second UN resolution was the safest legal course.
Ten days later Lord Goldsmith's final advice was published, but included no concerns about the legality of the war.
Michael Howard and Charles Kennedy said the advice raised fresh questions. But Mr Blair said the "smoking gun" had turned out to be "a damp squib".
The issue dominated the questioning when the three main party leaders appeared on a Question Time special.
Lord Goldsmith's 7 March advice was never shown to the Cabinet - instead, the 17 March advice was. It was also made public in an answer in the House of Lords. The war started on 20 March.
In the earlier advice, Lord Goldsmith raised possible legal arguments which could be made against the Iraq war.
He warned there were "a number of ways" in which opponents of the war could bring legal action.
"We cannot be certain that they would not succeed," he said, adding a second UN resolution might be the way of preventing such legal action succeeding.
Key questions he considered included whether the wording of previous resolutions on Iraq authorised military action.
But Lord Goldsmith's nine-paragraph written answer to Parliament on 17 March raised no such doubts, stating: "Authority to use force against Iraq exists" from previous UN resolutions.
Speaking on a special edition of the BBC's Question Time programme, Conservative leader Michael Howard said he believed the war was right, but that Mr Blair had lied.
On the same programme Liberal Democrat leader Charles Kennedy said the advice had had to be "absolutely dragged" out of the government and attacked Mr Blair's use of the phrase "damp squib".
"Go and describe these findings as a damp squib to the families of the service personnel who gave their lives in Iraq."
He said the Lib Dems still believed British troops should be aiming to return at the end of the year, even if the Iraqi government requested them to stay, although he said it would be possible to consider a request to be part of a UN peacekeeping force.
Mr Blair, asked about the issue by audience members, said: "It's not a matter of the attorney's general's advice because it's been shown that he advised it was lawful. Neither was it a matter of misusing intelligence.
"It is, however, a question of a difficult decision I had to take; Was it better to leave Saddam in power - or put him in prison? I think it was better to put him in prison."
Earlier shadow foreign secretary Michael Ancram told BBC News that Tony Blair should have disclosed the advice in full when MPs were discussing whether to go to war against Iraq.
Mr Ancram said: "He said that the attorney general's advice that was given was clear. I think he may have used the word unequivocal and hadn't changed.
"We now know from the publication of today's document that it was anything but clear - that it was qualified in many different ways and that in the end the attorney general had changed his mind."
Chancellor Gordon Brown sprang to Mr Blair's aid, saying he not only trusted the prime minister but he also respected him for his decision - and that he himself would have taken exactly the same decision in the same way.
But shadow attorney general Dominic Grieve said Mr Blair's position had become "untenable" after the revelations.
7 March: Early legal advice sent to Mr Blair
7 March: Hans Blix says Iraq accelerating co-operation
17 March: Final legal advice given to Cabinet
17 March: Advice revealed in House of Lords
20 March: War starts
Lord Goldsmith earlier issued a statement saying the advice backed up the government's position that the war was legal, and was simply a consideration of all the arguments.
BBC political editor Andrew Marr said the government could have avoided a "huge conspiracy" by releasing the legal advice on Iraq earlier.
He told BBC2's The Daily Politics: "I think it's one of the greatest own-goals ever in our secretive political culture. I think it has caused the government 100 times more trouble now than if they had released it originally."
Plaid Cymru's Adam Price, who has been leading a bid to impeach Mr Blair, called for his resignation, saying MPs who voted for war had been persuaded by "essentially fraudulent means".
Green principal speaker Caroline Lucas said: "Tony Blair has committed the gravest error that a prime minister can and so if he won't resign, then he must be impeached."