A cancer-causing chemical from bracken could be leaching into water supplies, scientists warned this week.

A British Ecological Society meeting in Lancaster was told that high levels of ptaquiloside (PTQ) had been found in wells on Danish and Swedish farms. And Venezuelan scientists said they had noticed a clear link between high levels of stomach cancer and bracken-covered countryside in their country. But experts said it was a theoretical risk because of tight water controls.

Lars Holm Rasmussen from the Royal Veterinary and Agricultural University found PTQ in two farm wells - one in Sweden and one in Denmark. "I found PTQ inside soil below bracken pants and I found it could enter drain water one metre below the soil surface. "On two occasions I found it in well water with drinking water," he said. The levels he found were 20,000 times higher than the suggested tolerable levels for environmental cancer-causing agents in Denmark.

Polluting water

He said it was a possibility that the same thing could be happening elsewhere. "I have proven that PTQ is mobile in the soil in the same way as nitrate. "In some cases it can pollute soil and drinking water," he said.

Bracken is known to cause cancer in cattle that have been grazing on it. According to Cancer Help UK, if bracken were to get into milk or the water supply, it is possible that it would be carcinogenic (cancer causing). But a spokesperson said this was very unlikely in the UK.

"In the UK, milk is produced by large dairies that get supplies from a wide area. Most of this would be unaffected by bracken. It is unlikely that any carcinogenic effect would be strong enough to affect those drinking the milk. "Similarly the mains water supply would not contain very much bracken contaminated water."

Debbie Hutchings from Cancer Research UK said: "Any research into possible causes of cancer is useful. "But for most of us, bracken is unlikely to be a major cause of cancer," she said.

A spokeswoman from Dwr Cymru Welsh Water said the company applied a considerable number of rigorous tests to drinking water to ensure that all the water it supplied complies with the exacting standards required by the Drinking Water Inspectorate. "Dwr Cymru Welsh Water is confident that its water treatment processes are robust enough to cope with multiple sources of water, including those from upland areas, to exclude any potential risk alleged to be associated with bracken," she said.