By Alex Kirby BBC News Online environment correspondent

The results of a three-year scientific experiment on the environmental impact of genetically modified crops in the UK are due to be published.

But Thursday's announcement will disappoint hopes of a resolution of the GM debate.

Protesters have uprooted GM crops

The results are expected to show that wildlife was harmed in field trials of GM oilseed rape and sugar beet varieties, while GM maize fields were much less affected.

Ministers will be unable to claim the trials give GM crops a clear green light.

The results will be published in the journal Philosophical Transactions Of the Royal Society: Biological Sciences.

The trials, known as farm-scale evaluations (FSEs), are the largest scientific experiment of their kind on GM crops anywhere in the world.

Gene flow

The government opted for them in 1999, to assess the impact on wildlife of plants genetically modified to tolerate certain herbicides.

Three crops were chosen: oilseed rape, sugar beet and maize, with the GM versions planted alongside their conventional equivalents for comparison.

Q&A: GM crop trials Monsanto departure pleases greens

The trials were criticised as too narrowly focused, looking at what the GM crops might do to farmland creatures, but ignoring other possible effects such as damage to consumers' health, cross-pollination with other plants, harm to soil organisms, and the long-term transfer of modified genes creating so-called superweeds.

There were protests against the FSEs, and crops in some trial fields were pulled up.

But the government decided it had enough data to make the experiment worthwhile and pressed ahead.

Monsanto pullout

On the eve of the publication of the trial results, anti-GM campaigners claimed a victory after the biotech giant Monsanto announced it was pulling out of the European cereal seed business.

The firm said on Wednesday its research centre in Trumpington, Cambridge, will close with the loss of about 80 jobs.

The company blamed the failure in the growth of a market in hybrid wheat seeds for the decision, saying it had "failed to materialise".

It said it was "streamlining" its operations.

But Friends of the Earth said Monsanto "set up the operation in Cambridge five years ago with the clear intention of introducing GM wheat and barley into Europe. This has been a pretty abject failure."

Consumer resistance

In the last few months, though, a series of reports has come out against the commercial planting of GM crops in the UK, at least for now.

In July the Cabinet Office Strategy Unit said they would bring little short-term benefit to the British economy, with lack of consumer demand limiting farmers' willingness to grow them.

It did say that future GM crops might offer benefits to both farmers and consumers.

Ten days later the government's GM Science Review Panel said the crops posed a very low risk to human health, but it expressed doubts about their possible environmental impact, especially on wildlife.

The government's chief scientist told BBC News Online precaution should prevail, and the existing moratorium on growing GM crops should stay in place.

In September the results of a nationwide debate showed "caution and doubt, through suspicion and scepticism, to hostility and rejection" towards GMs from the 40,000 people who took part.

The results of the farm scale evaluations are supposed to remain a secret until they are announced.

But press reports suggest they will say the GM oilseed rape and modified beet seem to do more harm to plants and insects than conventional crops, and should not be grown commercially.

Chemical row

The reports say maize does less harm, and so might win approval as a wildlife-friendly crop.

Opponents of GMs say the maize tests were invalid and should be repeated, because the herbicide used on the conventional control crop was atrazine, to be phased out by the European Union.

They say its replacement may well be more benign to wildlife, cancelling out the advantage which GM maize appears to enjoy.

The biotechnology industry says this is a red herring, arguing atrazine is used on only about half the maize grown in the UK, and the tests were to see whether it was better to apply a herbicide to germinating weeds or later in their growth.

So the stage is set for another inconclusive attempt to decide for or against growing GM crops in the UK. Neither side is holding its breath.