The results suggested that blood of an infected animal can transfer BSE-like disease to another long before it displays outward signs of the condition, and appeared to confirm the prudence of human blood donation controls introduced last year.

Scientists from the institute of animal health, based in Compton, Berkshire, and Edinburgh, took the rare step of publishing the work in a medical journal, the Lancet, within weeks of the discovery because of its importance. Only last month researchers based in London suggested that their work, using hamsters and mice, raised serious questions about how animals thought to be healthy and incapable of acquiring BSE could in theory pass the disease on.

Variant CJD, which has so far claimed 82 victims in Britain, is now widely regarded as BSE in human guise. In another development scientists in Paris today claim a breakthrough in the struggle to understand how BSE-like diseases attack the brain, leading to hopes that treatment for the incurable condition may be on the distant horizon.

The scientists involved in the sheep experiments fed animals with cattle brain infected with BSE and, before they displayed any symptoms, transfused blood into healthy sheep imported from BSE-free New Zealand. The one sheep to have so far shown signs of BSE began displaying the symptoms 610 days after transfusion. The donor sheep had eaten the BSE material 318 days before the transfusion but did not display signs of the condition for a further 311 days, indicating the blood had been infective halfway through incubation.

All UK blood has to be filtered so the white cells thought most likely to carry vCJD are removed. Plasma used in blood products is imported from countries where there is no evidence of vCJD. The measures cost the NHS £80m last year.

The government insisted last night this was a precautionary safeguard to counter a theoretical risk and there remained no evidence that vCJD "has ever been transmitted to humans through blood transfusion or blood products".

The scientists warned their report "suggests that blood donated by symptom-free vCJD-infected humans may represent a risk of vCJD infection among the human population".

They had no evidence of levels of infectivity of pre-clinical CJD cases. But the presence of BSE activity in sheep blood suggested it would soon be possible to identify which blood cells carried the infectivity. This could help test the effectiveness of blood donation controls and develop tests for vCJD based on blood samples.

Colin Bostock, the institute's director, said: "This work shows it was prudent to take action against a theoretical risk."

The implications of the work, along with that of the London team, are to be studied by the government's advisers on BSE this month.

The Paris researchers studied prions, molecules of a chemical called PrP, which are thought, in abnormal form, to be closely associated with BSE-like diseases. Healthy humans manufacture PrP in their bodies but its function has been a mystery.