Concern that pollen from genetically-modified (GM) plants can travel much further than the UK Government had thought is justified.

But the risk of it doing very much damage when it does spread far and wide need not be great.

Pollen is spread in one of two ways in a country like Britain - on the wind, or by insects. (It is also spread by fruit bats.)

How far it will travel by either method depends on what crop it comes from.

Pollen from maize - the crop studied in recent and controversial work by the National Pollen Research Unit - is designed to be spread over a long distance by the wind.

Dr Alastair McCartney, an expert on the dispersal of biological particles, said he was "not too surprised" at the report that maize pollen could travel beyond the 200-metre barrier the government requires round GM trial crops.

Dr McCartney told BBC News Online: "There is no zero risk.

"If you want to have no danger at all of pollination of other crops, then no barrier zone could prevent it.

"In theory, pollen can travel for hundreds of miles."

But Dr McCartney believes the 200-metre limit is "not unreasonable for most crops".

Degrading over time

"When pollen is dispersed, its concentration changes with the distance it travels. At 200 metres, it will not be very concentrated."

Time also takes its toll on pollen, which gradually loses its potency. But the rate varies with different species, and is also affected by the weather.

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Pollen which has travelled some distance always has to compete with local pollen, Dr McCartney says, which is another factor making it harder for it to pollinate plants it lands on.

"In the case of different species, cross-pollination is very difficult. There is only a very small chance of it."

But if the pollen lands on plants of the same species, or a closely-related one, then there is more chance of pollination occurring.

Even then, there may be other obstacles in the way.

"It depends partly on whether or not the species is self-fertilising", says Dr McCartney.

"There have been experiments with two crops of oil seed rape side by side.

"Cross-pollination of one by the other falls off in about 10 or 20 metres, because the plants fertilise themselves, so the pollen from their neighbours faces competition.

"So surrounding GM crops with plants of other species should significantly reduce the chances of cross-pollination."