The advice from English Nation is contrary to the impression given by ministers in the House of Commons earlier this week.

English Nature wrote to Prime Minister Tony Blair warning that crops genetically modified to be resistant to herbicides should not be grown commercially until research is fully completed.

Friends of the Earth said the letter was a "devastating blow" to the GM food industry.

And Shadow Agriculture Secretary Tim Yeo also said it was "devastating". He said he "fully supported" the call for a three-year ban.

"It would in our view be outrageous to overrule this advice."

Earlier this week William Hague warned that English Nature was calling for a commercial ban on some products.

But on Wednesday, Cabinet Office Minister Jack Cunningham told MPs the Tory leader had made "an error of interpretation".

Jack Cunningham denied English Nature supported a ban

Dr Cunningham said: "The opposition may be calling for that irresponsibly, damaging prospects for Britain, but they are totally misleading in saying that English Nature supports that call."

The letter from English Nature's chairman Baroness Young was intended to be a confidential briefing for the prime minister but was released after Dr Cunningham quoted from it in the Commons.

Dr Cunningham quoted a passage from the letter in which Baroness Young said: "We are not asking for a moratorium on commercial release of all genetically modified crops".

But the letter goes on to say English Nature was "very concerned about the effects that introducing herbicide tolerant crops would have on biodiversity".

It warns of the "disastrous" effects of previous attempts to make farming more intensive.

Baroness Young: "We cannot assure the public"

Baroness Young says: "Our advice to government has been that herbicide tolerant crops and insect resistant crops, not all GM crops, should not be released commercially until ... research has been completed and assessed."

In the letter, she welcomes the one-year voluntary ban announced by the crops industry but warns: "This will not give enough time for the research to be done, which we estimate will take at least three years."

The letter concludes: "It is important that English Nature can be in a position to reassure the public that the technology is environmentally safe, with decisions being made on the basis of good environmental science.

"We cannot assure the public about this currently."