But while it showed an increased incidence of leukaemia in children whose home address at birth was within 200 metres of a power line, the study's authors admitted they had not proved that proximity to the power lines was the cause.

"There is an association between childhood leukaemia and proximity of home address at birth to high voltage power lines," the study concluded.

The team, which included John Swanson the scientific adviser to National Grid Transco plc, said they had no satisfactory explanation for their results in terms of the power lines' magnetic fields directly causing the cancer.

"The findings are not supported by convincing laboratory data ... We emphasise again the uncertainty about whether this statistical association represents a causal relation," the study said.

Leukaemia is a cancer of immature white blood cells.

Research shows around 1 in every 2,000 children worldwide develops leukaemia before the age of 15, the disease usually occurs between the ages of 2 and 4 years old and the majority of cases can be successfully treated.

The study, the largest to date of the relationship between childhood cancer and power lines, began in 1997 and tracked the records of around 29,000 children in England and Wales -- including 9,700 suffering from leukaemia -- plotting the distance of their postal addresses at birth from power lines.

Gerald Draper, of the University of Oxford Childhood Cancer Research Group who contributed to the study, said the lack of a causal link should not discredit the study's findings.

"We say the results could be down to chance but they're statistically really strong. It looks as though they are real, we just don't know." he told reporters.