The food safety watchdog has pledged a relentless investigation into allegations of horse meat being passed off as beef in the UK after two British plants were raided and shut down as part of the inquiry into the scandal.

Andrew Rhodes, Food Standards Agency (FSA) director of operations, said they would keep pursuing an investigation into a meat processing plant in west Wales and into a slaughterhouse in West Yorkshire until there is "nothing left to find" after discovering evidence of apparent "blatant misleading of consumers".

Yesterday, meat processing plant Farmbox Meats at Llandre in Aberystwyth, west Wales, had records seized and were temporarily shut down pending the outcome of investigations into claims they supplied and used horse carcasses in burgers and kebabs.

The Food Standards Agency is a science and evidence-led organisation and we don't do things lightly. We have very good evidence to support the actions that we have taken. Indeed we interviewed people on that site near Aberystwyth yesterday and those people confirmed that they received the goods that we believed they received and they told us what they had done with them. We are very clear on what has happened there. That is why we have seized all the meat that is there, that is why we have seized all the paperwork and that is why we involved the police in Dyfed Powys.

An advert for a butchers in Cardigan in west Wales Credit: Helen Watling

The Welsh Government's Deputy Minister for Agriculture, Alun Davies, said: "Integrity and trust are essential in the food chain.

"I would be appalled if these allegations are proven.

"The Welsh Government is working closely with the Department for the Environment, Food and Rural Affairs (Defra) and the FSA to ensure this matter is dealt with swiftly and decisively."