Opponents of the technology say the results, which will be closely watched around the world, confirm their worst fears about the technology and repeated calls for a ban, or at least more research.

"These trials were a political fudge that did not begin to address the possible catastrophic effects that GM could bring about," Stephen Tindale, director of environmental group Greenpeace, said in a statement.

"For years the GM corporations have been claiming that their crops would reduce weed killer use and benefit wildlife. Now we know how wrong they were, Tony Blair should close the door on GM for good," Greenpeace said.

The results showed that in fields of herbicide-resistant GM maize there were more weeds and insects than in fields sown with conventional plants, but in fields of GM rapeseed and sugar beet, weed density and wildlife numbers tended to be lower than the conventional equivalent.

The findings have given the government a fresh headache over the issue as the research has failed to say whether GM crops should get the green light.

U.S. BIOTECH GIANT PULLS OUT

The report came just a day after U.S. biotech giant Monsanto MNSN.BO -- the company that provided the GM sugar beet for the UK trials -- said it was pulling out of the European cereal seed business.

Monsanto, which has pushed for transgenic crops to be raised in Europe for years, said it was closing a key crop research unit in Cambridge, southern England, with the loss of 80 jobs.

The move is seen as a blow to UK efforts to pioneer developments in biotechnology and prevent a so-called brain drain of scientific expertise.

British farming minister Margaret Beckett said the government would consider the results carefully before making up its mind.

"I have said consistently that the government is neither pro nor anti-GM crops - our over-riding concern is to protect human heath and the environment, and to ensure genuine consumer choice," she said.

But the UK's Consumers Association said GM crops would destroy the public's freedom to choose.

"Today's results have confirmed our concerns that commercialisation of GM crops in the UK will destroy consumer choice once and for all," it said.

The trials, which tested GM rapeseed and maize produced by Bayer CropScience, the UK arm of German biotech giant Bayer AG BAYG.DE and Monsanto 's sugar beet, did not investigate whether conventional or organic crops could co-exist safely alongside GM crops.

A full report by the government's Agriculture and Environment Biotechnology Commission (AEBC) is expected to report on this, and the question of liability, later this month.

MORE RESEARCH TO BE PUBLISHED

The Agricultural Biotechnology Council (ABC), which represents firms like Bayer and Monsanto, said the results showed GM crops had a positive contribution to make.

"These results confirm what the industry has long argued. The flexibility of GM crops allows them to be grown in a way that benefits the environment," ABC chairman Paul Rylott said.

Biotech industry group the Supply Chain Initiative on Modified Agricultural crops (SCIMAC) said: "The results themselves will take some time to digest in full, but it is immediately clear that the differences between the crops studied is governed above all by the crop type, herbicides and weed control practices involved, not by the use of genetic modification," it said in a statement.

But additional research papers published earlier this week by Britain's farm ministry failed to show GM crops in such a positive light.

In two separate studies, researchers found that bees carrying GM rapeseed pollen had contaminated conventional plants more than 26 kilometres (16 miles) away.