The BBC's Mark Turner: This new controversy comes at a particularly sensitive time

The Royal Society for the Protection of Birds (RSPB) had claimed Sir Robert May was at odds with the government after receiving a letter from him about the conduct of three GM plant experiments.

Our approach must be governed by good science, says Sir Robert

In the letter, published in the Independent, Sir Robert said: "I guess we really are in complete agreement, because I share your view that I 'do not see how ministers could contemplate giving permission for commercial release of the GM crops covered by this research until January 2003'."

The Chief Scientific Adviser has helped shape government policy

The RSPB claimed this contradicted the stated position of the government.

"Ministers have said they will review whether to allow commercial release of these crops every year," Dr Mark Avery, director of conservation at the RSPB, told the BBC.

"That does seem daft. The government has set up these trials and we believe ministers should say now that they will wait for the results and then make decisions."

BBC Science Correspondent Palab Ghosh: "Further research is needed"

But Sir Robert denied he was in any disagreement with a policy he helped to form.

"We are not going to give permission for commercial release until we are thoroughly satisfied and until all the appropriate field tests have been done. Where we differ from the RSPB and English Nature is that the science must set the time scale; there should not be some arbitrary moratorium.

"If these test are going to take six months, they take six months; if they take six years, they will take six years."

Sir Robert: This technology offers the chance to create a sustainable agriculture

The row surfaced as scientific correspondence was published in the journal Nature showing pollen from GM corn could kill the larvae of the monarch butterfly.

Dr Avery said such research reinforced the need for tests to be as extensive as possible. "This just adds to the alarm as it shows a new type of worry on these crops," he said.

"As time goes on - and this is why we need more research - we are actually getting more worried about the environmental effects of GM crops, not less."

Wildlife could be affected, says the RSPB

Sir Robert reiterated that the government would take a precautionary approach to the introduction of the new GM technology.