The study, involving almost 10,000 cases over 33 years, found the effect was small but statistically significant. There was also an effect on children living between 200 and 600 metres from the power lines who had a 23 per cent increased risk of leukaemia.

However, the researchers, led by Gerald Draper of the Childhood Cancer Research Group at the University of Oxford, were unable to offer any explanation for their findings, published in today's British Medical Journal. They ruled out electro-magnetic fields produced by the power lines as a cause as they would be too small to have any effect at a distance of 200 to 600 metres. About one child in 2,000 develops leukaemia before the age of 15 and about 4 per cent of the population live within 600 metres of power lines. Overall, the researchers estimate that proximity to the overhead cables may account for five of the 400-420 cases of childhood leukaemia occuring in England and Wales each year.

Dr Draper said: "These results are very surprising. Although previous studies have suggested some increase [in the risk of leukaemia] you have to be much nearer [within 60m]. No one would expect an effect at this distance.

"We don't think this is a direct effect of the magnetic field at 200 to 600 metres. There might be some other indirect, remote mechanism that we don't understand and we can't measure. Or the findings might be due to chance."

It was possible some characteristic of the populations who lived near the power lines, or of the area where they were situated, could account for the increase in leukaemia. The disease is 10 per cent more common in better-off families and varies by geographical area.

John Swanson, scientific adviser to National Grid Transco and a co-author of the study, said: "The findings strongly suggest something is happening but leave open what that something is. I personally tend to the view that it is some characteristic of the populations that the power lines pass through."

Another hypothesis is that pollutants in the air around power lines could become electrically charged, making them more likely to lodge in the lungs.

Dr Swanson said the failure of the UK Childhood Cancer Study to find any effect of power lines could be because it was too small.

Both authors said that there were no grounds for taking any action such as moving house on the basis of their results.

Dr Draper said the study, begun in 1997, was incomplete because the researchers had not measured the size of the magnetic field generated by the power lines and compared it with that from other sources. He said that he and his fellow researchers had decided to publish early after an environmental group accused them of suppressing the findings and leaked some details of the research last year. An editorial in the BMJ says magnetic fields generated by Britain's 4,500 miles of power lines are very weak, amounting to 1 per cent of the Earth's magnetic field "which affects all of us all the time. So it would be surprising if they caused leukaemia," it adds.

Professor John Toy, the medical director of Cancer Research UK, said: "People who live or have lived near power lines need not panic about this research.

"The triggers that cause childhood leukaemia are most likely a random course of events over which a parent has no control," he added.

Conflicting studies

* Swedish study finds threefold increased risk of childhood leukaemia in families living within 50 metres of power lines.

* Canadian study finds 80 per cent increased risk for children within 100 metres of lines.

* Analysis of nine studies finds doubling of childhood leukaemia in families exposed to magnetic fields of 0.4 microteslas (within 60 metres).

* International Agency for Cancer Research classifies extremely low frequency magnetic fields as "possibly carcinogenic".

* UK Childhood Cancer Study concludes electromagnetic fields "are not principal causes, if at all, of leukaemia in children."

* Oxford Children's cancer group concludes living within 600 metres of lines could account for five cases of leukaemia a year.