The government should reveal the date it first sought legal advice on the Iraq war, a watchdog body has said.
The government has refused to publish Lord Goldsmith's advice in full
The Foreign Office (FCO) had refused the request from the Lib Dems, saying it might "harm the frankness and candour of internal discussion".
But Parliamentary Ombudsman Ann Abraham rejected its reasoning and recommended the information be released.
Lib Dem peer Lord Lester said the FCO's continued refusal to reveal the date was "deplorable".
He intends to apply for the information under the Freedom of Information Act.
Although Lord Butler's inquiry found much of the pre-war intelligence was unreliable, the prime minister has said he acted in good faith and maintains that Saddam Hussein posed a threat.
Various groups have requested the attorney general's full advice on the legality of the conflict under the Freedom of Information Act.
But the government has refused, because it says that this would break the convention that legal advice to the government is not published.
The Lib Dems hope to narrow down the timetable to war - there have been suggestions US plans were passed to UK army bosses in October 2002.
Lib Dem Foreign Affairs spokesman Menzies Campbell said: "There must be a political explanation for the government's obduracy.
"One possible explanation is that as early as April 2002 the legality of military action was under active consideration."
He added : "It is not the national interest which is at stake here, it is government embarrassment."
Lord Lester made a formal complaint to the ombudsman after first submitting a written question in the House of Lords in April 2004.
Ministers claimed exemption from the Code of Practice on Access to Government Information on the grounds that releasing the date would "harm the frankness" of private talks.
But the conclusion to the ombudsman's report, made public by the Lib Dems, reads: "I have found that the code exemption cited by the department could not be applied to the information sought by Lord Lester.
"I therefore upheld his complaint and recommended that the information be released to him, with whatever other details might be necessary in order to put that information into context.
"I am disappointed that the department have found themselves unable to accept this recommendation."
A spokesman for the FCO told BBC News the department would not be complying with the ombudsman's advice.
"The FCO can't disclose the date because in doing so we would negatively impact on our ability to have frank policy discussions with our legal advisers in the future," he said.