The test also suggests CJD may be more infectious than thought and could be spread through routine surgery.

BBC Science Correspondent Pallab Ghosh: Disposable surgical instruments are expected to come into use

Until now it has only been possible to diagnose cases of new variant CJD, the disease linked to BSE in cattle, after death.

The test involves taking tissue from the tonsils and can be conducted on living people.

The findings could eventually mean scientists will be able to develop a test which could diagnose people with nvCJD as soon as they are infected.

The BBC's Pallab Ghosh: "The Department of Health has welcomed the study"

Research can now be done on material from tonsillectomies to estimate the extent of nvCJD - which has a long incubation period - in the general population.

But the findings also raise concern about infection caused by the rogue proteins that spread the disease and cannot be cleaned from surgical instruments no matter how thoroughly they are sterilised.

Professor Collinge: A new way to detect nvCJD

Writing in The Lancet, the scientists led by Professor John Collinge at the Imperial College School of Medicine at St Mary's Hospital in London, say they tested tonsil tissue from 20 patients in the late stages of suspected prion disease.

An abnormal form of the protease-resistant prion protein is thought to cause the breakdown of brain cells associated with nvCJD.

Prions reproduce in the tissues of the immune system, including the tonsils.

Because of the theoretical possibility that the disease could be spread by surgery to the infected organs Professor Collinge recommends that for such operations only disposable surgical instruments be used.

The infection can withstand high temperatures which make the usual sterilisation procedures ineffective.

Dr Stephen Dealer: Test is a "major advance"

Dr Stephen Dealer, from the BSE Research Campaign, said the findings would help to assess the risk from surgical instruments.

"This is one of the reasons why John Collinge's test may be so wonderful. It will give use some insight into just how big that risk is. This is a very important finding," he told BBC Radio 5 Live.

Professor John Collinge: "It offers the possibility of screening tonsils"

But Professor Collinge told BBC Radio 4's Today programme that people awaiting operations should not panic.

He said: "It is no cause for alarm but I think it is prudent to consider whether any precautions might be necessary."

Thirty-three cases of nvCJD in the UK and one case in France have been confirmed since 1996.

The researchers also tested tonsil, spleen and lymph node tissues from patients who had died of the disease.

They found that all immune system tissue obtained from dead patients whose CJD had been confirmed by brain biopsies contained the rogue prion.

Different progression

Tonsil biopsies of living patients found the tissue was positive for the prion in the three cases which were confirmed on death to have the disease.

Tonsil tissue examined during tests

The research also showed that nvCJD has a different progression from normal CJD and may spend longer in the immune system.

This suggests it could present greater dangers of infecting people through blood tranfusions, organ transplants and tissue-sharing.

They believe the difference in progression may be due to the suspected root of exposure - through eating BSE-infected meat.

This could suggest that the people who have developed nvCJD have immune systems which are particularly susceptible to nvCJD.

The scientists believe they may eventually be able to develop a test which is sensitive enough to detect prion infection at an early stage of infection.

Definite diagnosis

Professor Collinge, whose research is funded by the Wellcome Trust and the Medical Research Council, said: "This new test has already proven very helpful in the diagnosis of new variant CJD.

"While, unfortunately, we have at present no means to treat this dreadful disease, we can at least now provide a definite diagnosis at an earlier stage."