Police investigating a major cow passport fraud fear hundreds of animals that were too old for human consumption may have entered the food chain.

The probe is looking into the alleged use of falsified cattle "passports" to make tens of thousands of pounds in cash and subsidies.

It is feared that diseased animals could have been consumed by shoppers across Lancashire, the North West and beyond.

Police said the investigation into the alleged alteration of passports to change the animals' ages could represent the biggest case of its kind Britain has ever seen.

A Ribble Valley farmer was arrested as part of the investigation yesterday, following a joint operation between police, Trading Standards officers, Department for Food and Rural Affairs officials and the Rural Payments Agency, which oversees the payment of subsidies to farmers. They now fear that animals that were destined for culling, after they reached 30 months old, may have made their way into the food chain.

By law, animal passports have to be filled in and sent to the Defra-run British Cattle Movement Service (BCMS) every time an animal changes hands.

Police are investigating claims that cows without their own passports were bought because they were cheaper than legitimately registered beasts.

They are looking into whether they were then sold on for profit - either to rendering plants in return for government subsidies or for meat - after passports were allegedly falsified to match the particular animal.

Trading Standards have now warned that any such cases, if true, could seriously damage consumer confidence in British farming. Insp Bob Ford of Clitheroe police, who led an eight man police investigation, said: "We are investigating the trading in livestock which has occurred in such a way that could lead to hundreds, possibly thousands of offences."

Derek Smith, principal officer for animal health and welfare at Lancashire Trading Standards, said: "In terms of confidence in the British meat industry this could have serious consequences. Most farmers adhere to the rules on registration and movement and know they are there to restrict potential diseases. This is certainly the biggest potential breach of the regulations I have heard of in Lancashire and probably Britain."

Thomas Binns, livestock board chairman for the National Farmers Union in the North West, said: "Most farmers follow the rules to the letter. If however, there is eventually found to be a breach of the law, then the full weight of the law needs to be brought down."

The farmer was taken to Blackburn police station where he was questioned before being released without charge last night, pending further enquiries. He has been granted bail until March 7.