Chief Executive Officer Dr Derek Langslow and Chief Scientist Dr Keith Duff were giving evidence to the Science and Technology Committee.

English Nature supports large-scale field experiments, which are due to start this spring, since only these could provide the environmental research data needed, the committee heard.

Earlier, this year English Nature published a letter to Prime Minister Tony Blair, warning that GM crops resistant to herbicides should not be grown commercially until research is fully completed.

Jack Cunningham quoted from a letter from English Nature

Cabinet Minister Jack Cunningham had used excerpts from the letter saying English Nature was "not asking for a moratorium on commercial release of all genetically modified crops".

However, English Nature's Chairman Baroness Young went on to publish the full letter to clarify the group's position.

On Wednesday, MPs were told by English Nature that GM crops were likely to make farming more intensive than at present, which could threaten wildlife.

English Nature was not in principle opposed to GM crops, MPs were told, but the development of crops resistant to herbicides would allow virtually all weeds to be removed.

Dr Duff said: "There's pretty compelling evidence that existing intensive agriculture has had significant effects on wildlife in the countryside.

"Our concern is that genetic modification of traits such as herbicide tolerance will allow even more effective management of unwanted species."

The loss of weeds could affect animals which feed on them, therefore affecting the rest of the food chain, he added.

GM crops could also have a negative affect on the biodiversity of the countryside.

Dr Langslow said: "If you've got a situation where suddenly a crop you hadn't been able to grow in the west of England becomes possible to grow, you get a complete change of land use with all sorts of effects."

Dr Douglas Parr, Director of the environmental pressure group Greenpeace UK, told the committee he was wholly opposed to GM products.

Soya beans are one of the most common GM crops

He called for a change in the system by which ministers took advice from experts before deciding policy on GM issues.

He said the advisory committees began from a standpoint that genetic modification was desirable whereas instead they should be listening to the voice of public opinion.

Also appearing before the committee were representatives of the multi-national company, Monsanto plc, which has extensive interests in the development of GM products.

Ann Foster, responsible for Monsanto's public relations in the UK, said the company's high profile in the GM field meant it was suffering from an image problem during the current controversy.

Ms Foster confirmed that the company had held discussions with the government about GM-related issues.

She said: "We have had meetings with ministers ... all our meetings are a matter of public record."

The company had, for example, conveyed its concerns to Environment Minister Michael Meacher about suggestions that there should be a moratorium on GM development.