Several British TV stations and newspapers have reported that Attorney General Lord Goldsmith first believed the war was illegal, but then decided – possibly under political pressure – that it could be justified under existing UN resolutions.
The government has repeatedly refused to publish his full advice and has denied pressuring the attorney general.
A resignation letter written by the Foreign Office's deputy legal adviser, released on Wednesday under the Freedom of Information Act, appeared to add weight to reports that Goldsmith's opinion had shifted.
Elizabeth Wilmshurst, who quit her post because she was opposed to the looming war, wrote on 18 March, 2003 that "an unlawful use of force on such a scale amounts to the crime of aggression".
Two sentences in the letter were blacked out by the government. But Channel 4 News said it had obtained the missing material and said Wilmshurst had described how Goldsmith's opinion had changed.
Clare Short called the letter
devastating to Blair's claims
"My views accord with the advice that has been given consistently in this office, before and after the adoption of UN Security Council resolution 1441, and with what the Attorney General gave us to understand was his view prior to his letter of 7th of March," Channel 4 quoted the letter as saying.
According to several reports including by the BBC and leading British newspapers, Goldsmith warned the government on 7 March that an invasion of Iraq could be deemed illegal, but in a parliamentary written statement by Goldsmith ten days later, allegedly drafted with the help of Blair's office, said a war in Iraq would be legal.
Political opponents on Wednesday rounded on the government for censoring Wilmshurst's letter and accused ministers of a cover up.
"The government blacked-out that section not in the public interest, but in the government interest"
Foreign Office spokesman for the Liberal Democrats
"The government blacked-out that section not in the public interest, but in the government interest," said Menzies Campbell, foreign affairs spokesman for the opposition Liberal Democrats, who opposed the war.
Former Cabinet minister Clare Short, a bitter critic of the war, said the letter was devastating. "The bit that was blacked-out shows that the attorney general changed his mind twice in a matter of days before he gave advice to the Cabinet when he just said unequivocally, `My view is that there is legal authority for war', and kept from the Cabinet any suggestion that he had had doubts about it," she said.
A spokesman for the attorney general said Goldsmith had repeatedly made clear that his advice to Parliament was "his own genuinely held independent view, that military action in Iraq was lawful."