Britain’s most senior judges will be asked today for a ruling that could lead to the war in Iraq being declared an illegal "crime of aggression".

Until now, the courts have taken the view that they cannot rule on the Crown’s prerogative powers to wage war. But today the law lords will start hearing appeals by peace protesters who claim they were entitled to commit "criminal" acts in an attempt to prevent what they saw as the greater crime of launching an illegal war.

Nobody has been punished for aggression in international law since the Nuremberg Tribunal executed former Nazi officers in 1946. The new International Criminal Court does not yet have jurisdiction over the crime, partly because of difficulties in agreeing a definition of it.

But the Government has been told by its senior legal adviser that ministers could face such charges under English law.

In the Attorney General’s advice to the Prime Minister of March 7, 2003, Lord Goldsmith said: "Aggression is a crime under customary international law which automatically forms part of domestic law. It might therefore be argued that international aggression is a crime recognised by the common law which can be prosecuted in the UK courts."

Lord Goldsmith warned Tony Blair to expect opponents of military action to bring a case against the Government or military personnel. "We cannot be certain that they would not succeed," the Attorney General said. The first test case involves five peace campaigners who face criminal damage charges relating to RAF Fairford, Glos, and who are due to stand trial in Bristol later this year.

Another case involves 14 Greenpeace activists convicted of aggravated trespass after they occupied tanks at Marchwood military base in Southampton.

The third appeal has been brought by a woman convicted of aggravated trespass at RAF Fairford.

All of them claim that they are entitled to a defence under Section 3 of the Criminal Law Act, which says "a person may use such force as is reasonable in the circumstances in the prevention of crime".

The law lords are not being asked to rule whether the decision to take military action against Iraq amounted to aggression. Instead, the five campaigners due to stand trial in Bristol want them to rule that they can raise the argument in their defence.