Government officials refused to release the report on Saturday citing a "long-standing convention that advice to governments in office is not disclosed".
The announcement came shortly after Greenpeace asked for the confidential report by Attorney General Peter Goldsmith to be published.
Fourteen activists from the environmental organisation are due to go on trial next month for actions opposing the war and Goldsmith's report could be vital for their defence.
"We have written to the Crown Prosecution Service (CPS) asking it for the attorney general's full advice to the government on the legality of the war," said a spokesman for Greenpeace.
"We have given the CPS 24 hours to produce the full advice. Otherwise we will renew the request for the advice in court on the first day of the trial, set for March 9."
Greenpeace said lawyers for the activists - who are due to go on trial for illegally entering a British military base in February last year, in the run-up to the war - would prove their protest was justified by the need to avoid loss of life in Iraq.
But a spokesman for Prime Minister Tony Blair later said: "The attorney general's advice remains confidential because of the long-standing convention that advice to governments in office is not disclosed."
UK officials have been accused of
eavesdropping on UN officials
Greenpeace's request comes only days after charges of breaching the Official Secrets Act were dropped against a British intelligence translator who leaked plans of an apparent United States "dirty tricks" campaign targeting United Nations Security Council members in the run-up to the Iraq war.
Katharine Gun walked free on Wednesday after prosecutors said - without elaborating - that they would be offering no evidence against her.
Gun, a Chinese speaker sacked in June from her job at the Government Communications Headquarters, was charged in November under the Official Secrets Act 1989 of disclosing security and intelligence information.
She could have faced two years in prison.
Analysts said Blair's government might have feared that a public trial would have led to the publication of potentially embarrassing documents such as Goldsmith's confidential advice on the legality of the Iraq war.
Blair reacted angrily on Thursday when Clare Short, Britain's former international development secretary, alleged that British intelligence agents had eavesdropped on UN Secretary General Kofi Annan's conversations and that she had seen the transcripts.
Blair called Short's allegation "deeply irresponsible" and insisted that British intelligence agents always acted within the law.
Blair called Clair Short's charge
Since Short's statements, opposition leaders have asked Blair to come clean on the spying allegations and called for Goldsmith's advice to be published in full.
So far only his conclusion that the war was legal even though the United Nations had not given the green light has been published.
On Saturday, senior lawyer Lord Alexander de Weedon said publication of Goldsmith's argument was vital because it had allowed Britain to enter the war along with the United States in March last year.
De Weedon told BBC Radio 4's Today programme Goldsmith's advice was "the most important legal opinion of the last 50 years".
"Without it the war would not have gone ahead and 20,000 Iraqis would not have been killed.
"If there are good reasons it may restore some confidence (in the government)," he said. "If there are bad reasons it is important they should be confirmed."