The hold-up emerged yesterday when it was also revealed that the research is unlikely to start until next summer.

Final results into possible links with 'mad cow disease' will not be known until 2004 at the earliest.

The issue is urgent because milk from older cows those over 30 months which are more likely to have the disease is sold at thousands of supermarkets and corner shops. Any suggestion that milk could carry the BSE infectivity now or when the cattle epidemic was at its height in the 1990s will horrify millions of shoppers.

Government BSE experts believe there is a theoretical risk the rogue prion or protein linked to the infection can be passed on through milk.

But 13 years after the disease was first identified, official advisers admitted yesterday they are still in the dark about the threat.

Families who have lost loved ones to the human form of BSE last night condemned the delays in investigating a link with milk.

Frances Hall, whose son Peter died in 1996, said: 'BSE was discovered in cattle in 1989 but we are still in the dark about whether it was or is in milk.

'This is taking an awfully long time. These experiments should have started ten years ago. If they had, we might now have a definite answer.

'Everyone, including vegetarians, has gone on drinking milk and using milk products with the assurance there was no infectivity. If they now say there is something there, there will be an awful lot of angry people. It is a horrifying thought.'

The Government's previous advice has always been that milk is safe, based on experiments with mice.

However, there are concerns that these tests were flawed and not sensitive enough to detect transfer of the disease.

The delays in examining any link with milk were revealed at a meeting of the Government's expert committee, SEAC.

Members called for research more than two years ago but experts appointed by the Food Standards Agency have still not devised valid tests. SEAC chairman, Professor Peter Smith, admitted: 'There is still a question on milk because we don't have definitive tests.

'We do not even have the test for BSE in blood. There is a deficiency in the science here.' He said there was 'no doubt' the proposed experiments on milk could have started earlier.

'But the technology is evolving and the ability to pick up very low levels of the infective agent has improved enormously,' added the professor.

He stressed that, even if the BSE prion is in milk, there is no certainty this would cause the disease in humans. The FSA said that the science to detect the disease was advancing all the time and denied any unnecessary hold-up.

But one of the three panel members of the national inquiry into BSE has previously criticised delays in demonstrating the safety or otherwise of milk.

Professor Malcolm Ferguson-Smith, from Cambridge University, said: 'It is astonishing this research has not been done.'

It also emerged yesterday that the spread of human BSE may have peaked in 2000. So far, 129 Britons, mainly young adults, have contracted the disease.