An abattoir in Cheshire has denied being linked to a criminal scam to smuggle horses from Ireland to the UK.
A BBC investigation was told that horses with false papers have been sent to the Red Lion abattoir in Nantwich.
The abattoir's owner said no horse with false paperwork had ever knowingly been slaughtered there.
In a separate investigation, the Food Standards Agency (FSA) said it was looking at "inconsistencies with horse passports" at the slaughterhouse.
Speaking to BBC Northern Ireland's Spotlight programme, a man claiming to be part of a criminal gang said he had delivered horses to the abattoir on "multiple occasions" on forged papers.
He said some horses were unsuitable for human consumption.
The man, who did not want to be named, said: "[The horses] would come from anywhere, everywhere, good, bad or indifferent, it didn't matter.
"A lot of them were bought at farms around the country; people didn't want them any more or couldn't afford them.
"Some of the horses weren't in the best of condition, but to stimulate and get them up on their feet again you'd give them cortisone and Bute injections."
The man claimed the gang would give the horses fake passports so the animals were worth more when sold to slaughterhouses.
The owner of the Red Lion abattoir, High Peak Meat Exports, is licensed to sell horsemeat to Europe and said it openly and legitimately bought horses from Ireland.
It said it had never knowingly slaughtered any horse with false documentation and that it had turned away hundreds of horses because of inadequate paperwork.
In a statement, the firm said: "Every single horse passport is scrupulously checked for authenticity and at least one FSA official vet and meat inspector are present at all times.
"The decision as to whether the passport is authentic is for those government officials.
"The FSA are currently investigating a number of passport irregularities at Red Lion but all these passports were believed genuine by FSA at the time of slaughter," it added.
The company said it had previously written to both the Food Standards Agency and the Department for Environment, Food and Rural Affairs (Defra) because it was concerned about the authenticity of horse passports coming from Ireland.
The FSA said its investigation at Red Lion was mainly into inconsistencies with English-issued horse passports.
It said it did not want to comment further while its investigation was continuing.