The UK's attorney general cast doubt on the legality of the war against Iraq without a second UN resolution, a leaked document says.
Lord Goldsmith has said the war is lawful
In the document, which has been seen by the BBC, Lord Goldsmith tells Tony Blair a second resolution was the "safest legal course".
Ten days later his advice raised no such concerns about legality.
Lord Goldsmith issued a statement saying the document backed up the government's position.
Tory leader Michael Howard has queried the prime minister's honesty, but Mr Blair insists he has not lied.
And - in a BBC interview - he has received the public backing of chancellor Gordon Brown.
But the Lib Dems said the House of Commons would not have voted for war if the earlier legal advice had been known about.
The extract obtained by the BBC was sent to Mr Blair on 7 March 2003, a fortnight before the war took place.
In it Lord Goldsmith argues relying on the original resolution 1441 - which required Iraq to disarm - as authorisation for the use of force needed "strong factual grounds" that it had been breached.
The views of UNMOVIC - the UN monitoring, verification and inspection commission led by Hans Blix - and the IAEA, the nuclear inspection team led by Mohammed ElBaradei would be "highly significant", the attorney general reportedly said.
Mr Blair has steadfastly resisted pressure to release the full advice of the attorney general.
In the newly-leaked document, retyped from its original form, Lord Goldsmith says: "I remain of the opinion that the safest legal course would be to secure the adoption of a further resolution to authorise the use of force...
"I accept that a reasonable case can be made that resolution 1441 is capable in principle of reviving the authorisation in 678 without a further resolution...
"However, the argument that resolution 1441 alone has revived the authorisation to use force in resolution 678 will only be sustainable if there are strong factual grounds for concluding that Iraq has failed to take the final opportunity."
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
The legal advice was issued on the same day UN weapons inspector Hans Blix said more time was needed to disarm Iraq, that the country had accelerated its co-operation but that it could not be described as "immediate compliance"
The document concludes by saying: "If we fail to achieve the adoption of a second resolution we would need to consider urgently at that stage the strength of our legal case in the light of circumstances at the time."
In his statement, Lord Goldsmith said the document "stands up the case that the government has been making all along".
"What this document does, as in any legal advice, is to go through the complicated arguments that led me to this view.
"Far from showing I reached the conclusion that to go to war would be unlawful, it shows how I took account of all the arguments before reaching my conclusion."
And he said the war in Iraq was legal, adding that this was what he had said to government, to Cabinet and in public at the time.
Foreign Secretary Jack Straw told BBC News: "What changed between 7, when that advice was written, and 17 March when the attorney general came to his very clear decision that military action without a second resolution was justified was the circumstances."
This included evidence that had been given to Lord Goldsmith and others that Iraq was in breach of resolution 1441.
But it is understood the 7 March document, with its caveats, was never shown to a full Cabinet meeting.
Instead, Lord Goldsmith's later advice, described by Foreign Secretary Jack Straw as "unequivocal", was shown to the Cabinet on 17 March and made public in an answer in the House of Lords.
The war started on 20 March.
Mr Howard said: "It is now obvious from this legal advice that on 7 March 2003, the attorney general raised specific reservations about the legality of war in Iraq.
"But Mr Blair has said that the attorney general's advice to the Cabinet on 17 March was 'very clear' that the war was legal, and that the attorney general had not changed his mind. It is obvious that he did.
"So what the public must now have an answer to is this: what, or who, changed the attorney general's mind?"
The Lib Dems want a full public inquiry into the war and want voters to punish the Tories as well as Labour, as both supported the war, and choose their party instead.
Lib Dem foreign affairs spokesman Sir Menzies Campbell said: "If the House of Commons had known of the contents of this advice it wouldn't have voted to endorse military action.
"I strongly suspect that if every member of the Cabinet had seen a copy of this advice, others would have resigned, along with Robin Cook."
Former international development secretary Clare Short, an opponent of the war, said the leak would "confirm everything that I have been saying - it's very serious".
BBC political editor Andrew Marr said the document would greatly fuel suspicions but was not the "smoking gun" that opponents of the war were looking for.