The sign marks the edge of the exclusion zone

Dr Olga Kovalchuk, of the Friedrich Miescher Institute in the Swiss city of Basel, said the team had looked at mutations in microsatellite loci - the stretches of non-coded genetic material with no known function, sometimes called "junk DNA".

The study did not look at any mutating effect on genes. But Dr Kovalchuk said the team thought, because of the increase in mutations, that there could also be an increase in the rest of the genome.

Dr Yuri Dubrova, of the genetics department at the University of Leicester in the UK, said the plants grown on the contaminated plot had been exposed to relatively low rates of ionising radiation.

These, he said, "theoretically should not result in such a large increase in the mutation rate".

"The results of our study therefore point at as yet unknown effects of low-dose chronic exposure to ionising radiation, which make it substantially more mutagenic than previously thought.

"Future studies are clearly needed."

Uncertain

Dr Dubrova told BBC News Online: "I wish I knew whether our findings were applicable to other organisms, including humans.

"Probably you can't extrapolate them to human populations. But without more work we can't be sure.

Chernobyl is to close in December

"You certainly could not expect this sort of mutation rate after exposure at this low level, and it has to be explained.

"One possible explanation - and it is speculative - is that over a 10-month period the damage to the plant might be so small that the DNA repair system simply failed to recognise it."

Chernobyl's number four reactor was destroyed in an accident on 26 April 1986, and 31 people are officially listed as having died from the resulting radiation.

But unofficial estimates say the numbers killed by the indirect effects of the accident are anywhere from 15,000 to 30,000.

Ukraine says it will close the plant for good on 15 December. Western countries have promised $1.5bn to pay for the shutdown and for the completion of two new reactors elsewhere in the country.

Earlier this year, UK and Dutch experts said the restrictions in place on food grown around Chernobyl might have to remain in force for half a century, because the radiation was clearing so slowly.