A former government aide has claimed that the Attorney General changed his mind on the legality of the Iraq war shortly before military action began in March 2003.
Ministers have refused to publish Lord Goldsmith's advice that effectively gave the green light for force to oust Saddam Hussein from power.
But a resignation letter from Elizabeth Wilmshurst, deputy legal adviser at the Foreign Office, suggests that even as late as 7 March 2003 Lord Goldsmith was unconvinced the war would be legal without a second UN resolution.
By March 17, Lord Goldsmith had issued a written parliamentary statement detailing the legal case for war, and Mrs Wilmhurst resigned the day after.
In her resignation letter, which was obtained by BBC journalist under the Freedom of Information Act, she said an "unlawful" use of force on Iraq amounted to the "crime of aggression".
Channel Four News said a blacked out section of the letter read: "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 March 7.
"The view expressed in that letter has of course changed again into what is now the official line."
Conservative shadow Foreign Secretary Michael Ancram said Ministers were trying to save their own skin in view of the "damning evidence".
"I think they have scored an enormous own goal," he said.
"What they have done is not to decrease the amount of doubt and mistrust that there is about the way that the government handled the run-up to the Iraq war, but actually increased it.
"If there was a reason why the attorney general changed his mind between March 7 and March 17, then we should be told about it.
"What is happening at the moment is that the Government is trying desperately to cover its tracks."
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell said the blacked out part of the resignation letter had been done in the Government's interest, not the public's.
"The Government is severely embarrassed by the fact that there is continuing controversy about the legal advice given by the attorney general and the way in which he arrived at his final opinion," he added.
Earlier this month, former Foreign Secretary Robin Cook claimed Downing Street pressured the Attorney General to "colour" his legal advice on war with Iraq.
Mr Cook said Lord Goldsmith met Baroness Morgan, the head of Tony Blair's political team, and the Lord Chancellor Lord Falconer, before producing his final advice on the legality of invading Iraq.
Mr Cook said he discerned "a process of negotiation" between the aides in Downing Street and the attorney general.
Former International Development Secretary Clare Short, who resigned after the war began, said this latest revelation was "devastating".
"I didn't think there was anything left that would shock me but to have that in black and white and to know that is what he did is really shocking. He said he wasn't leant on, but he certainly turned head over heels a couple of times," she added.
The Foreign Office last night insisted it had the right to excise part of Mrs Wilmshurst's letter.