The BBC's Kirsten MacGregor: "GM pollen was in the wider environment"

The current isolation barriers mean GM crops must be more than 50 metres from conventional crops and 200 metres from organic fields.

But a monitoring exercise set up by the pressure group Friends of the Earth and the BBC's Newsnight programme found pollen travelled up to 20 times those distances.

Tests around a GM trial site at Watlington in Oxfordshire showed that airborne pollen had carried distances of up to 475 metres.

Pollen carried by bees had reached beehives at distances between 500 metres and 4.5 kilometres from the site.

The Environment Minister, Michael Meacher, told the BBC: "I accept we need to look at it again. They may need to be larger than they are at the present time.

"We need to look at this evidence which has been presented and make a decision.

Michael Meacher: "I accept we need to look at it again"

"We are already discussing with the organic farmers what the distances are that would satisfy them and be adequate in order to protect organic farming and conventional farming, both of which I believe we should do."

Although it has been known for a long time that pollen can travel many tens of kilometres, this is the first study looking specifically at the movement of GM pollen. It will be useful in assessing the impact of the farm-scale trials now underway in the UK. The work was carried out during June and July by the national Pollen Research Unit and a bee specialist.

There is new concern about the effects GM crops could have on the environment

With pollen from oilseed rape capable of surviving for up to three days, there is concern that it could fertilise compatible species.

"The commercialisation of these crops should not take place until we have enough information to make decisions about whether they will have an effect on biodiversity and in our countryside," said Brian Johnson of English Nature.

"We haven't reached that point yet and we won't really reach that point until the end of the field scale trials, so our view is that the decision needs to be made then, and not before then."

The study is the first to find GM pollen in beehives. Honey packers do their own tests for it, but none has yet been discovered.

GM pollen has been found in beehives for the first time

"Most of the honey comes from commercial beekeepers and they're already well used to moving their hives around to follow the various crops, and they will avoid GM sites," said Laurie Keys of the Honey Association,

"Of course, if it gets more widespread and extends beyond more than just a few trial sites, then it would be more of a concern."

Friends of the Earth is calling for the government to abandon the whole GM trials programme.

But Mr Meacher argued that continuing the trials was the only way to show if GM crops posed a risk to the environment or the human food chain.