Government plans to rid the United Kingdom of sheep deemed susceptible to scrapie - a disease related to BSE - might fail because a key scientific question remains unresolved.

Malcolm Ferguson-Smith, a former member of the BSE inquiry, said tests were needed to see if scrapie-resistant animals could carry the infective agent.

Without this knowledge, he feared the National Scrapie Plan could fail to eradicate the disease within the next 15 years.

The NSP is a voluntary scheme for owners of pedigree herds. It aims to identify rams whose genetic make-up renders them susceptible to the disease. Those animals are slaughtered or castrated so that over time the national flock comes to possess inherent scrapie resistance.

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The government announced last week that it is working on legislation for the compulsory screening of all sheep. Professor Ferguson-Smith said: "The rationale for this strategy depends on determining whether sheep resistant to the development of the disease are resistant to infection."

He argued that it was possible that scrapie-resistant sheep might resist the development of the disease but not infection. They might not develop symptoms during their lifetime, but could still carry the infection.

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Professor Ferguson-Smith said samples of spleen and lymph nodes from resistant sheep should be screened for signs of infection to resolve the issue.

Tests have already shown that scrapie-resistant sheep can contract BSE.