The right of a jury to return its own verdict against the run of the evidence and in the face of the facts was first established at the famous trial of two Quakers, William Penn and William Mead, more than 350 years ago.

In their case the judge ordered the 12 jurors to convict the defendants of "unlawful and tumultuous assembly" for preaching on a Sunday afternoon in the City of London.

When they refused, he locked them all up overnight without food or water and fined them 40 marks. It took the intervention of the Court of the King's Bench to free the jurors from prison in a case that firmly enshrined the independence and protection of the juryfrom the state.

By Victorian times, juries were regularly exercising their power to acquit those caught red-handed stealing sheep because they knew the punishment for a conviction, hanging, was excessive and disproportionate.

The English common law adopted the same principle under the doctrine of lawful excuse allowing juries to exercise clemency when a small crime was committed in order to stop a much greater evil. Parliament later recognised the same principle when it passed legislation such as the Criminal Damage Act 1971. It was that statutory defence of lawful excuse which the Kingsnorth Six relied on yesterday to secure their acquittals.

They follow in the footsteps of other green campaigners who have also used the same defence to avoid conviction. In 2000, a jury at Norwich Crown Court cleared Greenpeace activists, including the former director Lord Melchett, of criminal damage for cutting down GM crops.

Since then, anti-nuclear protesters have achieved similar success in defeating prosecutions for criminal damage and trespass. But, two years ago, the House of Lords rejected the defence of lawful excuse when peace campaigners smashed up vehicles used to load bombs on to American B-52s based at RAF Fairford in Gloucestershire. The defendants had argued that the entire war against Iraq was a crime of aggression and a much greater evil. The law lords neatly sidestep that argument by ruling that the protesters could not use this defence because that while aggression by the state is a crime under international law, it is not a crime under domestic law.