If they were to do it over again, Phil Pritchard says, they would bring less stuff. He and Toby Olditch were carrying "silly amounts" when they broke into RAF Fairford on the eve of the Iraq war: bolts and screws to be placed inside the B-52s' engines, pictures of smiling Iraqi children to be stuck on to the payload doors, toothbrushes and stamps to be used in prison, and flashing headbands in the hope that they would look too ridiculous to be shot.

Somewhat inevitably, they were arrested before they got anywhere near one of the bombers. "At that stage," says Mr Pritchard, "I think the word that went through our heads was: arse." Instead of preventing the jets from taking off for Iraq, the two men were charged with conspiracy to commit criminal damage, remanded in custody for three months, and told to expect a jail sentence of up to 10 years.

But while their mission may have been, in Mr Olditch's words, "a bit Keystone Cops", it was not criminal. That was the remarkable verdict of a jury at Bristol crown court this week, which unanimously acquitted the two men, having accepted their defence that they were acting to prevent the US air force planes from committing war crimes.

Their action was reasonable, they had argued, because other means of legitimate protest had failed. The result, according to their lawyer, is the closest an English court has come to accepting that war crimes were committed by Britain and America in Iraq.

"I knew our legal argument was good, that we should be acquitted," said Mr Pritchard, 36, "but I didn't necessarily expect that to happen." They had prepared themselves for a five-year sentence. "So, yes, I was surprised, and delighted. And very impressed by the jury."

As a jury verdict in a crown court, the ruling does not set a direct legal precedent, but its implications may be significant for future protesters. The pair - who inevitably became known as the B-52 Two - were not permitted to argue in court that the war was illegal, after a Lords ruling judged that the decision to go to war was protected by crown prerogative and could only be tested by the International Criminal Court.

Instead, their defence relied on two key arguments: that they were acting to prevent a crime, in their case what they perceived as potential war crimes against Iraqi civilians, and to prevent criminal damage to Iraqi property.

"In strict legal terms," says Mike Schwartz, their solicitor, "[the verdict] is not significant; what is significant is that you had 12 jury members who unanimously found these defendants not guilty, and that really is a clear barometer of the public's views of the lawfulness of what the government did three or four years back.

"Because what Phil and Toby feared would take place was so close to what actually happened, in effect the jury implicitly was saying that war crimes were committed. And the judge, who had the power to say these defences weren't relevant, allowed them, which is in some way a judicial endorsement."

The two men, both self-employed carpenters from Oxford, are not obvious anti-war protesters. Mr Pritchard's father is a retired RAF officer; Mr Olditch had considered joining the Royal Navy; neither considers himself a pacifist. Mr Olditch, 38, became politicised during the Falklands war and belonged to the anti-Nazi movement; Mr Pritchard, two years younger, says his interests have always lain principally with environmental concerns.

But having pledged to take all necessary steps to prevent war, and feeling other means of protest had been ignored, they drove to the Gloucestershire base on March 18 2003, the night before the bombing assault on Baghdad began, cut the outer fence, slid under the razor wire, crawled through nettles, and headed for the runway before they were apprehended.

The ruling is not unprecedented: in 1996 four women who battered the nose cone of a BAE Systems Hawk jet bound for Indonesia were acquitted after showing the planes had previously been used in attacks against civilians in East Timor. In July last year, five Irish peace campaigners were found not guilty after damaging an American transport jet at Shannon airport.

Two other cases are still active, involving two men and a woman who had also broken into RAF Fairford in separate incidents around the same time as Mr Pritchard and Mr Olditch.

The judgment should not be seen as a vandals' charter, says Mr Schwartz - crucial to the jury's decision was the argument that their action was reasonable under the particular circumstances at the time. He cites the "dodgy dossier", allegations of an agreement earlier between George Bush and Tony Blair to go to war, the "skewed vote" in parliament. "All those showed this was a grotesque failure of a democratic process to work, and in those circumstances extreme measures were justified."

Toby Olditch, 38, and Philip Pritchard, 36, both from Oxford, expressed delight and relief after a Bristol crown court jury unanimously found them not guilty of conspiring to cause criminal damage at RAF Fairford in Gloucestershire on the eve of the invasion of Iraq in March 2003.

Speaking outside the court after a verdict whose implications are potentially hugely significant for both the government and anti-war protesters, Mr Pritchard said: "I am delighted. It is a great relief - and a huge vote of confidence for anti-war protesters - that a jury were convinced that our actions were lawful."

He added: "This verdict sends out an important message."

Mr Olditch told the Guardian: "We are overjoyed and thankful for the good sense of the jurors and for the wonderful support we have received. But hundreds of thousands of Iraqis have suffered from the government's actions."

The two men, who came to be known as the B-52 Two, pleaded not guilty to conspiring to cause criminal damage when they tried to disable the bombers to prevent them from attacking Iraq. They argued that they acted to prevent damage to life and property in Iraq, and war crimes by aggressors. It was the second time they had been tried for the same offence. The first, in October 2006, ended in a hung jury, after 12 hours of deliberation spread over three days. Had they been convicted, they could have faced up to 10 years in jail. Two similar cases are awaiting re-trial, as a result of hung juries, at Bristol crown court.

The protesters argued that war crimes would be committed in the bombing as the B-52s carried cluster weapons - which scatter unexploded "bomblets" that kill and maim civilians. They argued that the aircraft were also armed with "bunker busting" bombs tipped with depleted uranium that fragments and spreads radioactive toxins harmful to civilians.

The prosecution conceded only that delaying the bombers would have prevented civilian casualties since it would have allowed people fleeing cities more time to escape.

The court heard that the pair entered the base on March 18 2003, the day before bombing commenced at the start of the Iraq war. They were armed with bottles of red and brown coloured liquid along with bags of nails and staples which would have been poured into the planes' engine bays. The pair were arrested after being spotted by patrolling Ministry of Defence police.

Before sending out the jury, Judge Tom Crowther said it was crucial to ignore wider issues relating to Britain's decision to go to war. "I remind you, you do not have to decide the legality of the decision to go to war."