A Sussex campaigner today won a landmark legal ruling over her conviction for damaging genetically modified crops.

Rowan Tilly, of North Laine, Brighton, took the case to the High Court to challenge her conviction for aggravated damage to a GM oilseed rape crop in August 1999.

Mrs Justice Rafferty ruled today that public order laws could not be used against campaigners who damage GM crops if no one else is in the field at the time.

Campaigners were jubilant, saying the ruling could mean no charges being brought against a number of people who have been arrested at GM demonstrations.

Miss Tilly said: "I am over the moon about it. When the ruling was delivered we all broke out into broad grins. It's got important implications for all GM pullers who believe these crops are harmful to the environment."

Both anti-GM activists and the Crown Prosecution Service were challenging conflicting decisions by lower courts in different parts of the country.

Miss Tilly, 43, and two others were charged with digging up plants in Cambridge with the intention of disrupting the lawful activity of growing the crop, contrary to Criminal Justice and Public Order Act.

A stipendiary magistrate convicted Miss Tilly in June last year, fined her £250 and ordered her to pay £250 legal costs. Cambridge Crown Court dismissed her appeal in November and ordered her to pay a further £300 costs.

Both Cambridge courts rejected Miss 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.

Miss Tilly had better luck at Weymouth Magistrates' Court, Dorset, in June this year when she and six other anti-GM activists were cleared of similar public order charges.

Today's hearing quashed Miss Tilly's conviction in Cambridge and dismissed the CPS appeal against the Weymouth decision.

Mrs Justice Rafferty said the protesters could not be convicted in either case because the 1994 Act required that other people had to be engaged, or about to be engaged, in a "lawful activity" on the land for there to be an offence.

The purpose of the 1994 Act was to prevent intimidation and the disruption of people engaged in lawful activity. In these cases no one was working on the land during either of the crop raids involving Ms Tilly.

The judge said: "I have no hesitation in concluding presence is necessary before the offence under this section can be made out."

After the hearing Miss Tilly said: "This ruling means they can no longer use this public order charge against us, although they can still use criminal damage or civil trespass."

The Director of Public Prosecutions is considering appealing to the House of Lords.