The Guardian in its investigation into the Europe-wide scandal over horsemeat sold as beef has managed to trace a key source of the meat in the UK.

A Cheshire-based slaughterhouse known as Red Lion and owned by the Turner family, who slaughter and cut horsemeat and who own a cargo handling company in Dundalk, Ireland, has been found to be a key source of the meat finally delivered to the Anglo-Irish Beef Processors — the leading processor of cattle in Europe with about 50 million Europeans thought to be its consumers each week.

The ABP bought some of its meat from a Dutch businessman called Willy Selten, who ran a meat cutting plant in the town of Oss, south of Rotterdam in Holland.

A spokesman for the Turner family has however insisted that all their horsemeat deliveries to the Dutch businessman had been properly labelled as horse and were legal.

Moreover, the ABP told The Guardian that the meat from Selten was sourced for ABP not directly but on occasion by Norwest Foods, a Cheshire-based trading company. Norwest Foods was set up by Ray MacSharry Jr, son of the former Irish agriculture minister and European commissioner and, the Guardian has established, a former employee of Larry Goodman, the Dundalk-born beef baron and property magnate who owns ABP.

Polish workers for Selten interviewed by the Guardian and Dutch TV have claimed they were cutting up and mixing horsemeat, which was being delivered from slaughter-houses in the UK and Germany, with several years old defrosted beef.

The workers said the meat would be so old that it was sometimes "green". They also said they “had to tie towels around their faces” to stop themselves being sick.

The workers further referred to “out-of-hours shifts paid in cash” during which they mixed the different meats and said “relabelling of horsemeat had been going on for up to five years”.

Selten was arrested in May 2013 by Dutch authorities “on suspicion of fraud and false accounting, when official tests on boxes of meat labelled as beef taken from his factory found horse DNA in 21 per cent of them”.

On its part, the ABP blamed the adulteration in its chain “on rogue managers” at its Silvercrest factory in the border area of Ireland. The ABP said the managers in question were “buying in frozen meat for burger-making from traders who were not on the list of suppliers approved by its customers”.

In September, ABP and Norwest Foods announced that they had reached a confidential financial settlement, and Norwest apologised for unwittingly selling horsemeat to ABP Silvercrest. ABP refused to respond to queries about where the Selten meat had ended up.