Foreign Secretary Jack Straw is answering claims the attorney general changed his mind about the legality of the Iraq war just before it began.
Lord Goldsmith has denied being 'leaned on'
Lord Goldsmith initially thought the war was illegal without a new UN resolution but backed the invasion 10 days later, it is claimed.
The claims are made in a Foreign Office lawyer's resignation letter.
Opposition parties are now pressing Mr Straw for a full explanation about the issue in Parliament.
The revelations came in a censored part of ex-Foreign Office lawyer Elizabeth Wilmshurst's letter, obtained by Channel 4 News.
Before 7 March: Lord Goldsmith believed war not legal without new UN resolution, claims Elizabeth Wilmshurst
7 March: Lord Goldsmith tells Tony Blair it would be safer to have second resolution
17 March: Parliamentary answer from Lord Goldsmith says war legal without new resolution
18 March: Tony Blair makes case for war ahead of MPs voting for military action
20 March: Invasion of Iraq begins
Conservative spokesman Dominic Grieve, who has tabled Thursday's urgent question, said Lord Goldsmith was fully entitled to change his mind.
But he told BBC News: "The difficulty is that that change of mind is against the backdrop of the prime minister having massaged the truth when he appeared before Parliament."
Liberal Democrat leader Charles Kennedy said the government had "huge questions" to answer.
He urged the government to publish the attorney-general's full legal advice on the war - "and if necessary be damned".
Such a move would improve the "quality of debate in the forthcoming general election," Mr Kennedy added.
Ex-Foreign Secretary Robin Cook urged ministers to clear up the "running sore" and suggested possible MPs might not have voted for war had they known of Ms Wilmshurst's concerns.
"It is very difficult to avoid the conclusion that what changed... was the discovery that we were not going to get the second resolution," said Mr Cook.
Asked by reporters about the claims, the prime minister's official spokesman repeatedly said it was convention that governments did not discuss the attorney general's advice.
Ex-Defence Minister Lord Gilbert defended the government's refusal to publish Lord Goldsmith's full advice.
He said: "They [the public] know the attorney general has said the liberation of Iraq was legal and that's good enough for me and I think it should be good enough for the whole country."
He said many countries had believed Iraq possessed weapons of mass destruction and other nations involved in the war had also decided it was legal.
'Crime of aggression'
Ms Wilmshurst resigned as deputy legal adviser to the Foreign Office on the eve of the invasion on 20 March because she did not believe military action was legal.
A censored version of her resignation letter has been obtained under freedom of information laws by the BBC News website and other news organisations.
Ms Wilmshurst argued the Iraq war amounted to a "crime of aggression".
Part of the letter is blanked out - the Foreign Office says law officers' advice is exempt because of professional privilege.
Channel 4 News, without giving a source for its information, published the missing section.
It suggests the attorney general, the government's chief law officer, changed his mind twice about the war's legality.
Channel 4 quoted Ms Wilmshurst saying that Lord Goldsmith originally had agreed with Foreign Office lawyers that the war was illegal without a new UN resolution.
He was reportedly more equivocal on 7 March, telling Tony Blair in a letter it would be safer to seek a new resolution.
"The view expressed in that letter has of course changed again into what is now the official line," said Ms Wilmshurst.
That was a reference to Lord Goldsmith's parliamentary answer on 17 March saying the war was legal without a new resolution.
A spokesperson for the attorney general told BBC News: "More of the same questions about process have been raised.
"What matters is that as recently as 1 March this year, the attorney general made very clear to the House of Lords, that the view set out in his parliamentary answer of 17 March 2003 was his own genuinely held independent view, that military action in Iraq was lawful."