Two anti-nuclear protesters who entered a dockyard planning to disarm one of Britain's Trident submarines with an axe were yesterday cleared of conspiracy to cause criminal damage.

Sylvia Boyes, 57, from Keighley, West Yorkshire, and River - formerly Keith Wright - 45, from Manchester, admitted that they plotted to damage HMS Vengeance while it was docked at Barrow-in-Furness, Cumbria, in November 1999.

But they denied criminal damage, claiming their actions were justifed because nuclear weapons were immoral and illegal under international law. They argued politicians could not be trusted with Britain's nuclear arsenal, so civilians had to act to stop disaster.

The Manchester jury's verdict, which follows a string of acquittals in cases of sabotage against military equipment was welcomed last night by anti-nuclear campaigners, who described it as highly significant.

"The jury has given the clearest possible decision legally, factually and morally, on the continued possession by Britain of weapons of mass destruction and where governments fail us, of the necessity of direct action", Gareth Peirce, lawyer for Sylvia Boyes.

David Mackenize of Trident Ploughshares said it was the first time the prosecution had brought a conspiracy charge, a notoriously broad indictment, against members of the group. "It will be a superb encouragement for activists", he said.

At Manchester crown court, the judge, Rhys Davies, had earlier told jurors that the two protesters could be seen as "thoroughly decent and ideal istic people" doing what thought was right. But however genuine the activists' beliefs, ideals formed no defence against a criminal damage charge.

Ms Boyes, a former lollypop lady, and River, an Open University lecturer with a master's degree in high energy nuclear physics, were found dressed in wetsuits near the submarine on November 23. They were carrying hammers, an axe and six cans of aerosol varnish.

The protesters, members of the direct action group, Trident Ploughshares, said they aimed to disarm HMS Vengeance which was equipped to carry one quarter of Britain's nuclear arsenal.

A spokeswoman for Trident Ploughshares added: "At last a precedent has been set for English people to follow their conscience and declare Trident illegal.

"This will pave the way for further legal direct action to disarm submarines."

The case is not the first example of direct action protesters walking free from court. At Manchester crown court last year, a jury acquitted two women of spray painting a nuclear submarine at Barrow with anti-war slogans and were unable to reach a verdict on a second charge of criminal damage. The prosecution asked for a retrial.

In 1999, a sheriff in Greenock, Strathclyde, handed down not guilty verdicts against three women charged with causing £80,000 damage to Trident submarine computer equipment at a naval establishment on Loch Goil.

In 1996, a jury at Liverpool crown court acquitted two women charged with causing £1.5m damage to a Hawk fighter jet at a British Aerospace factory.