Stray and abandoned dogs from Spain may have been used in pet food and animal feed.
Spanish authorities have not ruled out the possibility that protein or fats from the carcasses could have been used in processed food for human consumption.
The British Food Standards Agency says it is liasing closely with its Spanish counterparts in the investigation.
Bodies of dogs and other animals from animal sanctuaries, vets, zoos and farms, which should have been incinerated, were apparently taken by a criminal gang, that then processed them to create protein and fats that could be sold on.
Evidence has been found at warehouses and processing plants in Galicia and Salamanca.
Last year, police found a warehouse filled with 15 tons of dead stray dogs which they believe were destined to be processed into animal feed, in Galicia and other places in the north of Spain.
The environmental group of the Guardia Civil, Seprona, has sent samples of commercial pet food to the Anfaco-Cecopesca laboratories in Vigo, Galicia, after a judge received reports from someone from inside the industry.
According to laboratory tests performed in Salamanca, fat samples destined for animal feed had DNA traces of both sheep and dog.
Justin Kerwell of the pressure group Viva, which campaigns against meat eating, has written to supermarkets in the UK asking for them to carry out tests.
‘It may only be a matter of time before dog, rat and perhaps even cat meat is found in British processed food or farmed animal feed.
‘British supermarkets have been selling horse meat to consumers for years without knowing it, so what else has been on sale? They will have no idea unless they specifically test for it’.