Three men have been arrested over allegations that horsemeat was mislabelled as beef.
Two men were arrested in Llandre, near Aberystwyth. Dyfed Powys Police said the men, aged 42 and 64, were held at Farmbox Meats Ltd on suspicion of fraud.
A 63-year-old man was arrested at the Peter Boddy Licensed Slaughterhouse in Todmorden, West Yorkshire, in a simultaneous operation and also on suspicion of fraud.
All three were being held at Aberystwyth Police Station and interviewed by police and staff from the Food Standards Agency.
Catering supplier Rangelands Foods in Ireland has withdrawn its burgers because some of its products contained "significant levels" of horsemeat, it said.
The company found that some of its frozen burgers had between 5 and 30 percent horsemeat and that affected products had been distributed to wholesalers and caterers in the UK.
Approvals for both Farmbox Meats and Peter Boddy Licensed Slaughterhouse were suspended by the FSA, meaning neither company was operational.
They were investigating the mislabelling of horsemeat that was being sold as beefburgers and kebabs. Peter Boddy had supplied Farmbox with meat.
Rangelands Foods withdraws products
Andrew Rhodes, director of operations at the FSA, said: "I ordered an audit of all horse-producing abattoirs in the UK after this issue first arose and I was shocked to uncover what appears to be a blatant misleading of consumers."
Horsemeat infected with the hazardous drug phenylbutazone - or bute - have been found in eight horse carcases slaughtered in the UK.
Three of those tested may have entered the food chain in France, while another three were shipped abroad but are not at risk of being eaten by people.
Dame Sally Davies, chief medical officer for England, said that while bute does pose a risk, it is very low.
She said: "If humans have eaten contaminated meat there is a very low risk. In the recent results the amount of bute is between trace and 1.9mg per kilogram.
"A person would have to eat between 500 and 600 100percent horsemeat burgers in one day to get a treatment does of phenylbutazone."