GM protestors have broken into a research centre and destroyed part of a plot of genetically modified wheat.
GM crops have often been targeted by protesters
The group said 60 of its members got into the
Jealott's Hill Research Centre, near Bracknell, Berkshire, at 0430 BST on Sunday.
The demonstrators say they cut through two wire fences, but were not challenged by either security or the police.
The facility is owned by Syngenta - one of the world's largest agro-chemical companies, and one of three UK-based companies trialling GM crops.
Taking direct action to decontaminate trials ourselves is the only way we can stop GM before it is too late
A Thames Valley Police spokesman said security staff at the Jealott's Hill had confirmed the raid.
"Some time during the night, somebody has cut through a fence, broken in and trampled some crops down," said the police spokesman.
"It is not clear whether the company has made a complaint.
"They have requested that we attend, so when an officer goes there they may make a
complaint of criminal damage, if there is one."
The group, which does not have a name, claimed the crop was about to pollinate and spread GM material into the surrounding countryside.
Liz Snook, one of the protesters who took part, said: "It's like wandering into an episode of the X-Files out there.
"Syngenta are using our land as a laboratory despite the courts repeatedly upholding the position that these crops are a very real threat to property and the environment."
Another group member, Pipa Gallop, said: "The public has said a resounding 'no' to GM crops, but the government isn't listening.
"Taking direct action to decontaminate trials ourselves is the only way we can stop GM before it is too late," she said.