Top scientists have accused the British government of continually ignoring repeated requests for a nationwide sheep survey for signs of BSE, and of putting the public's health at risk as a consequence.

A study of the national sheep flock for evidence of the brain wasting disease, which is thought to be linked to the deadly human condition vCJD, should have been started years ago, according to Professor Roy Anderson, an epidemiologist at London's Imperial College, and Professor John Collinge, the director of the Medical Research Council's prion research unit.

Furthermore, they added, the research already carried out has proved so flawed that any potential problem has been overlooked.

The two experts have petitioned the government to expand its work into the potential spread of BSE in sheep, as well as in cattle, and have spoken about the role of the similar disease scrapie in the animals. They claim that BSE might not have shown up in sheep to date because it is masked by scrapie.

Anderson is quoted in the Sunday Times newspaper as saying: "Scrapie is found in only a few per cent of sheep. The fraction with BSE will be even smaller, so to detect it you need to sample thousands of sheep brains - but Defra (the Department for the Environment, Food and Rural Affairs) has looked at only 180. We have made this clear to Defra to no avail, which I find really very disturbing."

Collinge meanwhile first approached the Tory government in 1996, with a demand for widespread sheep testing put to colleagues on the government's Spongiform Encephalopathy Advisory Committee (Seac). He raised the issue with ministers when nothing was done, but still no programme was initiated.

Two years ago, he published a paper in The Lancet medical journal warning: "The possibility that BSE may have been transmitted to sheep has caused concern. BSE that has been transmitted is clinically indistinguishable from scrapie."

Collinge has also campaigned for research into the possibility that there are different strains of BSE, each with a different level of infectivity.

This weekend's comments came hard on the heels of Defra's revelation that a five year, £217,000 (US$310,785) study on sheep brains appeared to be fundamentally flawed. Scientists were actually analysing bovine tissue.

Margaret Beckett, the environment and rural affairs minister, has responded to the criticism by announcing a complete review of the government research into BSE organised by her department.