The British government today rejected fresh demands to release the Attorney-General's advice on the legality of the war in Iraq.
Environmental group Greenpeace is demanding access to Lord Goldsmith's advice to ministers so it can defend 14 activists in court following an anti-war protest last year.
But Downing Street refused to disclose the Attorney's advice, saying it must remain confidential.
The move comes after the case against former intelligence officer turned whistleblower Ms Katharine Gun was dropped after her lawyers asked to see the advice.
A Downing Street spokesman said: "The Attorney-General's advice remains confidential because of the long-standing convention that advice to governments in office is not disclosed."
Greenpeace said it was disappointed by the decision, but it would continue to press for the advice to be made public.
"This is very unfortunate but we are going to court to get this," a spokesman said.
"We never expected Tony Blair just to hand this over but we will ask for it in court. It is in the interests of getting a fair trial for these 14 defendants.
"If the Prime Minister is truly interested in democracy and giving people a fair trial this advice must be published."
The Greenpeace activists occupied tanks at the Southampton military base in February last year. Throughout their case they have argued their actions were necessary to prevent the loss of life.
Two anti-war protesters charged with offences following a demonstration at RAF Fairford in Gloucestershire are also to make a formal request for access to the advice.
Ms Gun was charged with breaching the Official Secrets Act after leaking a top secret e-mail disclosing United States plans to bug delegates at the United Nations Security Council ahead of a crucial vote in the run-up to the conflict.
Her lawyers speculated the case was dropped because they had demanded the Government disclose advice it received from the Attorney General - advice ministers have repeatedly refused to make public.