Exact grid references for the fields are among information put on the Department of the Environment, Transport and the Regions' Internet site on Monday.

A unit spokeswoman defended the decision to reveal the locations, amid protests from farmers and commercial groups behind the trials.

She said: "The government is very committed to being open about these trials."

The BBC's Margaret Gilmore: "The Government has decided not to keep trial sites a secret"

The four farm-scale trials of GM oilseed rape will begin this autumn at Market Rasen, and Glentham, both in Lincolnshire; Bingham, Nottinghamshire and Hemel Hempstead in Hertfordshire. A total of 75 test sites are set to be in operation by next year.

Last month Greenpeace claimed to have undermined the whole GM test process after its supporters allegedly destroyed a field of modified maize at Lyng, Norfolk.

But the new sites will not be given any extra security. Downing Street said it was a matter for local police forces, who "will be on hand to deal with any difficulty".

Roger Turner, head of the British Plant Breeders Association, said that although information on the sites' locations should not be kept secret, explicit details should not have been revealed.

"I'm all in favour of openness and transparency, but the difficulty is that environmentalists have lost the argument and are determined not to let the trials go forward to show that there are environmental benefits from these crops," he said.

He said the BPBA had recommended to the government that it did not publish the sites' specific locations, and that other parts of Europe, such as Germany and France, revealed only the county in which the trials were being carried out.

Farmer Ronald Duguid: "It would be simpler if they were kept quiet"

Farmer Ronald Duguid, whose fields were recently attacked by Greenpeace, has agreed to a second test on his land. But he said it would be "simpler" if the locations were not revealed.

He said he understood people's concerns about he trials, but thought they would prove to be "beneficial".

But Greenpeace said it will continue to campaign against the test sites, which would include "non-violent direct action".

The group's food campaigner, Jim Thomas, described the sites as "irresponsible", he said they were "releasing genetic pollution into the environment".

Trials are 'a farce'

"The market and the public have said no to GM crops, and the government's research has shown they will contaminate organic food and other farms," he said.

Greenpeace has been working with communities around the country and recently commissioned a MORI poll in which 63% of those questioned said they would oppose a GM farm-scale trial being planted near them.

Another environmental group, Friends of the Earth, called the trials "a farce".

Campaigns director Liana Stupples said: "The government's policy is to plant field after field with GM crops and see if anything goes wrong. This isn't science - this is creeping commercialisation."

Consumer complaints

The trials have already been condemned for being based on what one MP called "incompetent" scientific standards.

Alan Simpson, the Labour MP for Nottingham South, told BBC Radio 4's Westminster Hour: "All the scientific benchmarks currently in use are at best haphazard and at worst incompetent."

At the same time, the National Consumer Council has warned that poor labelling of GM foods is leading consumers to believe that they are not being given vital information.