Cases against protesters could be reviewed
Rowan Tilly was one of a group of campaigners charged with aggravated trespass and interfering with the "lawful activity" of growing genetically-modified (GM) crops.
The group was found guilty by a court in Cambridge in one incident but was acquitted by magistrates in Weymouth, Dorset.
But Judge Mrs Justice Rafferty ruled that the protesters could not be convicted in either case because there was no-one working on the land at the time of the protest.
Anti GM-protesters say the ruling could mean charges pending against a number of protesters will have to be reviewed.
She said she was overjoyed at the ruling: "I'm relieved because if that ruling hadn't been the way it was, I would be looking at a prison sentence.
"Also it means there's going to be a chance for justice for people pulling up crops in the future."
Ms Tilly, who said she had four charges pending, said it meant future protesters would get their chance to defend their actions before a jury.
'Fair trial'
"People can have a fair trial in front of a jury and argue that they are acting in the public interest," she said.
Ms Tilly and her lawyers say she should have been charged with criminal damage.
But they believe the police are reluctant to go for this option because the protesters could face trial by jury.
Ms Tilly said: "Aggravated trespass is an 'easy' charge for gaining convictions without a jury in a magistrates court.
"The alternative charge, criminal damage, allows a defence of public interest, and we've seen this defence used to acquit people before a jury in Norwich Crown Court."
She said the purpose of the act was to prevent the intimidation and disruption of people engaged in lawful activity.
"I have no hesitation in concluding presence is necessary before the offence under this section can be made out," the judge said.
Ms Tilly was involved in a protest at Lime Farm, Great Chishill, Cambridgeshire, in August 1999.
She was charged with aggravated trespass under the public order act after protesters dug up plants and dumped them at the headquarters of AgrEvo at East Wynch, Norfolk.
Ms Tilly was convicted by a district judge in June 2000 and fined £250 and ordered to pay £250 legal costs.
She lost an appeal against the conviction last November and was ordered to pay a further £300 in costs.
Both courts rejected Ms Tilly's contention that, as nobody was working on the land at the time the GM crops were pulled up, there could be no offence of aggravated trespass.