"We have written to the Crown Prosecution Service 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 that the defense attorneys of the militants, going on trial for illegally entering a British military base in February last year, in the run-up to the war, will prove that their protest was right to avoid loss of life in Iraq.
The organisation's request comes only days after spy charges were dropped against a British intelligence translator who leaked plans of an apparent US "dirty tricks" campaign targeting UN Security Council members in the run-up to the Iraq war.
Katharine Gun walked free Wednesday after prosecutors said -- without elaboration -- 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 the government of Prime Minister Tony Blair may have feared that a public trial would have led to the publication of potentially embarrassing documents such as Attorney General Peter Goldsmith's confidential advice on the lawfullness of the Iraq war.
Blair reacted angrily 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.
He called Short's allegation "deeply irresponsible" and insisted that British intelligence agents always acted within the law.
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.
Saturday, Lord Alexander de Weedon, a member of the Queen's Council, said that publication of Goldsmith's argument was vital because it allowed Britain to enter the war along with the United States in March last year.
Calling Goldsmith's advice "the most important legal opinion of the last 50 years", de Weedon told BBC Radio 4's Today programme:
"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."