The diseases, including variant Creutzfeldt-Jakob Disease (vCJD), currently cannot be detected by scans, and are normally only confirmed after death.

The test, reported in the British Medical Journal, has been developed by government researchers working at the US Agricultural Research Service in Ames, Iowa.

It works by detecting the presence of abnormal proteins called prions, which have been linked with the diseases.

Fears about the possible transmission of vCJD are so strong in the US that the Food and Drug Administration has barred people who visited the UK frequently between 1980 and 1996 from donating blood.

So far 47 people have died from confirmed vCJD cases in the UK.

Blood test

The test works by mixing blood samples containing special antibodies with manufactured prion fragments which have been made fluorescent.

In disease free blood, the antibodies will bind to them. However, if the abnormal prions connected with the disease are present, there will be fewer antibodies left over to bind to the artificial proteins.

This means that there will be more fluorescent proteins left to detect.

The test has been used successfully on similar spongiform illnesses in sheep, elk, deer and hamsters.

More research is now being carried out to ensure the test is reliable enough to use on humans.

At the moment, scientists in Britain use brain or glandular samples, taken after death, to confirm the spongy brain structures which characterise the illness.