GM crops could be potentially harmful to the environment, scientific trials ordered by the government have found.

Campaigners against GM food claimed victory after the scientific trials indicated there could be significant impact on biodiversity.

The results found some types of GM crops require less herbicide than conventional crops but others required more.

These conclusions will now be passed to the government's committee on releases into the environment.

The committee is set to decide the UK's position on whether GM crops should be approved for commercial use.

Environment secretary Margaret Beckett defended the decision to go ahead with the test programme.

"We persisted with this research despite the activities of some anti-GM campaigners, including serious attempts to destroy the trial sites," she said.

"So I am very pleased that the results are now available - we have said all the way through what have been fairly difficult years that they would provide valuable additional information to test the potential impact of growing and managing these crops on farmland wildlife."

Despite the prime minister's enthusiasm for GM, Beckett insisted that no decision had yet been taken.

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

Shadow environment secretary David Lidington called for further scientific research.

"The evidence published today would not justify a decision to authorise the commercial growing of GM crops," he said.

"These results deserve careful study. I shall want to hear the views of other scientists on the findings published by their professional peers.

"The fact that the impact of different GM seeds on wild plants and invertebrates varies so markedly shows that we must proceed with scepticism and caution.

"There should be no blanket approval. Any variety of GM seed proposed for licensing should be subjected to rigorous field trials and risk analysis."

Lidington added that policy on GM should be "driven by reliable scientific evidence and a rounded consideration of our national interest".

The outcome from scientist tests, however, failed to give the decisive answers some had hoped for.

Dr Les Firbank, co-coordinator of the project said: "The results of these farm scale evaluations reveal significant differences in the effect on biodiversity when managing genetically herbicide-tolerant crops as compared to conventional varieties."

He also issued a note of caution - stressing that more research would have to be undertaken before GM could be deemed risk-free.

"One of the key points to remember is that the results are only applicable to the three crops studied, and only under the regimes of herbicide usage which were employed," he said.

"Each new application of GM crop technology must be looked at on a case-by-case basis, using a rational evidence-based approach."

The announcement came as Monsanto, the American biotechnology firm specialising in genetically modified foods, revealed it is withdrawing from the European seed cereal business.

The firm is set to close its operation in Cambridge with the loss of 125 jobs.

Number 10 described the announcement as "a matter for the company" and said the government would be "guided by the science and protection of the environment".

The trials, which formed part of a three-pronged review of GM, were set up by the government in 1999.

They were ordered after English Nature, the government's wildlife watchdog, raised concerns about the possibility of crops polluting ordinary fields and organic farms.

Some ant-GM groups claimed the programme was a political fix to buy time in order to persuade the public of the merits of wholesale GM use.

But since then most of the high street supermarkets have banned products containing genetically modified material - leaving a question mark over their viability.

To compound matters, the European Union has imposed a ban on a weedkiller called atrazine which was a core part of the test programme.

Former environment minister Michael Meacher, the man who ordered the tests, has already warned that the programme has been effectively invalidated as a result.

And the Liberal Democrats claimed the government could not give the go-ahead for GM crops in the UK on the basis of the report.

"Every step of the way, official research has shown the public need much more reassurance. No responsible government would permit the commercial planting of GM crops under current circumstances," said Andrew George, the party's rural affairs spokesman.

"The public doesn't want it, supermarkets won't stock the stuff, there are major environmental concerns and there are no benefits to our farmers. The question must now be - why on earth would the government say 'yes' to GMs?"

Anti-GM campaigner and Labour backbencher Joan Ruddock described the results as "another nail in the coffin" for GM crops."Even if all crops had passed or failed the tests, other factors would still mitigate against growing GM crops commercially," she said.

"There is no shortage of cereal production in Europe and no economic case has been made.

"The precautionary approach must prevail and a case by case approach for commercialisation must be resisted.

"Monsanto's decision to abandon its European based research indicates its lack of interest in crop improvement by conventional means which offers the best hope for developing countries."

But experts suggested that the data would remain valid, although it may need to be "recalibrated" to allow for the use of alternative pesticides.