Pet food containing beef was found in the same area as cattle on a farm in Reading, Berkshire, and another in Kent in July last year.

This was not revealed at the time, to allow government vets to check whether the problem was more widespread.

Officials at the Department for Environment, Food and Rural Affairs (Defra) said movement restrictions were introduced on the farms and the cattle concerned would not be slaughtered for human consumption.

It is vitally important for public confidence and the continued decline of the BSE epidemic that the feed ban should remain the subject of rigorous enforcement Elliot Morley, Animal health minister

They refused to release any further details as criminal proceedings may follow from their investigation.

Animal health minister Elliot Morley said "Advice from the Food Standards Agency and our own veterinarians indicates that the public and animal health risks arising from this incident are low.

"But intra-species recycling is undesirable and goes against the specific advice of the Spongiform Encephalopathy Advisory Committee.

"It is vitally important for public confidence and the continued decline of the BSE epidemic that the feed ban should remain the subject of rigorous enforcement."

Regulations allow pet food containing processed animal proteins to be held on a farm, but say it must be securely packaged and stored to prevent contamination of livestock food.

The penalty for breaching the regulations is a maximum fine of �2,000 or two years in prison.