Supermarkets are investigating hygiene practices at a number of Welsh abattoirs after undercover BBC reporters posing as trainee vets claimed they found carcases contaminated with faecal matter.

The journalists secretly filmed in three abattoirs – Cig Calon Cymru in Cross Hands, Dunbia at Llandbydder and HMD, a small family plant in Swansea – and worked alongside Meat Hygiene Service inspectors. They claimed they found carcases that had been health-marked by inspectors still contaminated with faeces.

Jane Downes, MHS veterinary and technical director, said the agency took “very seriously any failure to maintain high standards”. The abattoirs each said they complied with best practice.

Sainsbury’s, M&S and Somerfield, which are supplied by the plants, said they were investigating.