The furious international row over the risks of depleted uranium weapons is set to flare up again with the revelation that a single atom of uranium inside the body is enough to trigger cancer. Scientists from a government-funded medical research laboratory at Harwell in Oxfordshire have produced the first direct proof that a single alpha particle emitted by uranium can damage human cells. The damage, they say, is a crucial step in the development of tumours.

Arguments over the health hazards of depleted uranium (DU) have been raging since the UN Environment Programme announced earlier this month that eight out of 11 sites in Kosovo were contam inated with radioactivity. Nearly one million rounds of ammunition containing 300 tonnes of DU have been fired in the Balkans and Gulf wars over the last 10 years, mostly by US forces.

The new evidence will reinforce the growing chorus of demands for a ban on the use of DU in weapons, at least until the health risks for soldiers and civilians have been fully investigated. Last week the European Parliament voted 626 to 394 in favour of a moratorium while an independent study into the potential health risks was carried out. Although Nato and the British Ministry of Defence continue to insist that there is no evidence that any ill health has been caused by DU munitions, politicians, war veterans and scientists maintain there is a problem, particularly with excess cancers and leukaemias. Tomorrow the World Health Organisation is sending a team of experts to Kosovo to examine whether there are any links between civilians exposed to DU and cancers.

The new British study was conducted by the Radiation and Genome Stability Unit at Harwell in association with Mount Vernon Hospital in London. Groups of human blood cells were exposed to a single alpha particle in the laboratory and left to divide a dozen times or more.

Researchers found that 25% of the daughter cells had distinctive patterns of broken and bent chromosomes. This effect, christened "radiation-induced genomic instability", is thought to be part of the complex chain of biological events that can end up as cancer. "This work shows directly for the first time that even a single alpha particle can induce genomic instability in a cell. That may be important in assessing risks of cancer from alpha-emitting radionuclides in the body," said the Harwell unit's director, Professor Dudley Goodhead.

"It suggests that even the smallest amount carries some, very small, risk. However, for materials such as low-activity uranium it may well be that the radiation is less harmful than chemical effects of the metal in the cell."

Although alpha particles are not a very penetrating form of radiation, when inside the body they can do significant harm to any living cells they happen to pass through. They are emitted by plutonium and other radionuclides as well as uranium.

Last week, the Ministry of Def ence admitted that traces of plutonium could also be present in DU weapons, left over from processing by the nuclear power industry. That, experts pointed out, inevitably increased the risk because plu tonium emitted more alpha particles than DU.

Mike Thorne, a uranium specialist with AEA Technology, a spin-off company from the UK Atomic Energy Authority, also in Harwell, thought that the new study strengthened the need to find out how much DU had found its way inside people during the conflicts in the Gulf and the Balkans.

But he agreed with Goodhead that the chemical effects of DU could be even more dangerous than its radiological impact. "It is a toxic heavy metal," he said.

"It would be reasonable to put a moratorium on its use as a munition until we have investigated the amounts to which people have been exposed."

Most radiobiologists have dismissed the much highlighted suggestion that soldiers exposed to DU in Kosovo in 1999 could already have developed leukaemia as a result. It was much too soon after the exposure for the disease to be diagnosed, they said.

But Sue Roff, a radiation res earcher from the Centre for Medical Education at the University of Dundee, has dug up evidence that suggests the radiation-induced leukaemias can develop surprisingly quickly. The first cases were discovered in Hiroshima and Nagasaki in 1947 and 1948, just two or three years after the US destroyed the cities with atomic bombs.

"In studies of patients who have received radiation therapy, the period of greatest risk for developing leukaemia has been reported as two to five years after exposure.

"I am aware of at least six men, most in their 20s, who were diagnosed with leukaemia within five years of their participation in the UK nuclear weapons tests in the 1950s and 1960s," Roff told the Sunday Herald. "In Japan, leukaemia was the first malignancy that appeared among the survivors and this fact triggered a major research study which is still in operation today.

"I would have thought that the appearance of leukaemia among soldiers selected for their fitness for active duty would trigger similar studies as part of the government's duty of care."