Tens of thousands of people could die from eating lamb products contaminated with BSE if the cattle disease has transferred to sheep, a scientific study published today warns.

Although there is no evidence that sheep have been infected with bovine spongiform encephalopathy (BSE), scientists believe there is a strong possibility that it has happened, raising fresh fears over food safety.

In their assessment of the human health risks from BSE in sheep, researchers from Imperial College London calculated that if sheep were infected as many as 150,000 people could die because of contaminated organs entering the human food chain.

This is three times higher than the "worst case" estimate for the maximum number of people that the same scientists reckon could die from eating beef products contaminated with BSE – although only 113 people have so far developed the human disease, vCJD.

The scientists also calculated that the risk from lamb could be reduced by 90 per cent if existing control measures were tightened so that all the internal organs of sheep were banned from human consumption.

Their warnings about the health threat from BSE in sheep rest on two main assumptions: that the disease has indeed passed from cattle to sheep and that over the years it has continued to spread from one sheep to another.

The risk assessment published today in the journal Nature says nothing about whether either of these assumptions is likely, but scientists believe both are highly plausible.

Professor Neil Ferguson, who led the Imperial College study, said: "In some ways I'd be surprised if BSE wasn't found in the sheep population."

It is known that contaminated feed caused the BSE epidemic in cattle, that the same feed was given to sheep in the 1980s – albeit in smaller amounts – and that sheep can develop BSE with symptoms similar to scrapie, a brain disorder of sheep that is harmless to humans.

Experiments have also shown that when the disease gets into sheep it behaves slightly differently to BSE in cattle in that it infects a wider range of tissues at an earlier stage in the animal's life – making BSE in sheep potentially more dangerous to consumers than an infection of cattle.

Finally, BSE in sheep could behave like scrapie, where the infectious agent is transmitted relatively easily between animals. This is not the case with BSE in cattle where, because of bans on contaminated feed, the epidemic is now in sharp decline. The ability of sheep BSE to transmit "horizontally" between animals would create a growing epidemic with the infectious agent spreading insidiously within the national flock of 20 million animals, its presence masked by the thousands of cases of scrapie reported each year.

The Department for the Environment, Food and Rural Affairs began a programme of sheep screening this month involving tests on 20,000 sheep at abattoirs and a further 3,000 sheep that died before going to slaughter.

A spokesman said: "One of the problems is that you can screen lots of sheep and find the presence of scrapie, but right now we can't differentiate between the two.

"A lot of work is going on to try to validate a practical test which will do that."

In the absence of a quick and reliable test that can distinguish sheep BSE from scrapie, scientists do not know whether sheep now have the disease, and the risk is more than "theoretical". But the study demonstrates just how serious the health risks to consumers would be if scientists did detect BSE in sheep. Professor Ferguson and colleagues conclude that sheep BSE could now pose a greater threat to human health than the cattle disease. In the worst case, they estimate that up to 150,000 people could die from eating sheep products contaminated with BSE.

Professor Ferguson explained: "The current risk from sheep could be greater than that from cattle, due to the more intensive controls in place to protect human health from exposure to infected cattle, as compared with sheep."

Existing bans on the consumption of sheep offal are not as extensive as those for bovine offal. The rules state that only the spleen of sheep should be removed from animals younger than 12 months. The skull, brain, eyes, tonsils, and spinal cord of sheep older than a year are also banned – but not lymph nodes or intestines, which are also known to become infective.

Last August, Dr Richard Kimberlin, a former government adviser and expert on BSE and scrapie, warned in an article in The Independent that the existing sheep offal ban was illogical and inconsistent.

He said: "We know now that several tissues from BSE- infected sheep, including lymph nodes, pose a greater risk than the same tissues from infected cattle."

The Imperial College researchers found that banning all internal organs of sheep would reduce the health risk to humans by 90 per cent. But they offer no advice on whether the Food Standards Agency (FSA) should extend the existing ban on sheep offal to include, for example, intestines or lymph glands. Azra Ghani, a member of the Imperial College team, said: "That's something for policy makers and I know it's being considered by the FSA at the moment. These things have cost implications and it's really a policy decision on whether this risk is sufficient to warrant further controls."

The FSA said it had no immediate plans to extend the current offal ban and was not advising the public against eating lamb. However, the agency is in the middle of a consultation exercise, which could lead to a change in policy.

The agency, which funded the Imperial College study, said the research "contributes to current thinking about the theoretical risk of BSE in sheep" but added that it was based on "limited data and uses a wide range of assumptions".