Natural gamma rays may be responsible for around 40 cases of childhood leukaemia in the UK each year, research suggests.

Scientists found a small but significant link between the risk of the disease and exposure to gamma radiation from environmental sources.

Higher levels of gamma radiation were associated with a greater likelihood of children developing leukaemia.

The findings contradict the widely held belief that very low radiation doses have little or no effect on cancer rates.

They follow separate results published last week suggesting that low dose X-rays from a type of hospital scan can triple the chances of children developing leukaemia or brain cancer.

Gamma rays are essentially a highly penetrating form of invisible light that form part of the natural background radiation everyone is exposed to.

The main sources of natural gamma rays are radioactive isotopes, or atomic variants, of uranium, thorium and potassium. These can be found in the soil, rocks, drinking water and even building materials. Another source is cosmic radiation from outer space.

Study leader Dr Gerald Kendall, from the Childhood Cancer Research Group at Oxford University, said: "We found a statistically significant correlation between natural gamma-rays and childhood leukaemia.

"What is new in our findings is the direct demonstration that there are radiation effects at these very low doses and dose-rates.

"In terms of preventing childhood cancers caused by natural gamma-rays, there's not a lot you can do.

"We have estimated that about 15% of the 500 or so cases of childhood leukaemia which occur annually in the UK are due to natural background radiation.

"Natural gamma-rays account for about half the dose (of background radiation) reaching children's bone marrow from all sources. So they account for approaching 40 childhood leukaemias a year."

Natural gamma ray dose rate varied in different parts of the UK, said the researchers writing in the journal Leukaemia.

The highest average exposure of 120 nanograys per hour (nGy/hr) was found in South Yorkshire, Cornwall, the Isles of Scilly and the Scottish Borders.

Powys in mid-Wales, Dorset and Wiltshire, had the lowest measured rate of 70 nGy/hr.

The study is the largest ever conducted on links between childhood cancer and background radiation.

Dr Kendall's team analysed tens of thousands of cases from the National Registry of Childhood Tumours, a complete record of UK childhood cancers.

The scientists looked at radiation exposures for almost 27,500 children diagnosed with cancer between 1980 and 2006, including more than 9,000 with leukaemia. These were compared with exposures for almost 37,000 cancer free children.

Cumulative gamma radiation exposures from birth to diagnosis were estimated for different regions.

The researchers found a 12% increase in the risk of childhood leukaemia for every millisievert of natural gamma rays entering the bone marrow. Millisieverts are used to measure radiation absorption by biological tissue.

No significant associations were discovered between natural gamma rays and other childhood cancers. Nor was a link found between any kind of cancer and levels of radon, a radioactive gas produced by certain rocks.

Co-author Professor Richard Wakeford, from the University of Manchester, said: "Radiation protection measures assume that even low doses of radiation pose some, albeit small, risk of cancer.

"Naturally occurring gamma-rays provide an ever-present, very low-level source of exposure to radiation, but this very large epidemiological study suggests that even at these very low levels there is a very small risk to health."

Last week another team of British-led scientists reported a tripling of leukaemia and brain cancer risk in children linked to CT (computed tomography) head scans.

The research, published in The Lancet medical journal, led to calls for greater efforts to ensure use of the X-ray scans is justified.