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.
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.
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.