Most supermarkets in the UK are thought to have been affected (Picture: Getty)

British supermarkets may have ‘unwittingly sold rotten meat’ to customers for years, an investigation has discovered.

Ready meals, sandwiches, quiches and other produce on shelves in Tesco, Asda, the Co-op, Morrisons and Marks & Spencer were undergoing urgent checks on Wednesday night.

Food maker Oscar Mayer, which supplies to Aldi, Ikea, Sainsbury’s and Subway, was also affected.

They were all alerted to the fiasco after an investigation by Farmers Weekly said rotting pork had been mixed in with fresh meat ahead of processing.

Rotting pork is alleged to have been mixed in with fresh meat (Picture: Getty)

It was also alleged that visibly off hams were regularly being ‘washed’, and frozen meat was sometimes thawed out on the factory floor of the firm, which cannot be named for legal reasons.

A source told the magazine the most rotten product would end up in hospitals, on patients’ plates.

The supplier is now the subject of a criminal investigation to establish the extent of the scandal.

But there is no suggestion that any of the processor’s customers were aware of what had been happening.

The ‘criminal practices’ are alleged to have taken place for at least two decades and likely beyond 2020, as auditors who visited the site were deceived.

Another source detailed how workers would ‘push stuff onto lorries and getting rid of them’ during surprise checks.

Former employees described the company’s operation as ‘one big fiddle’, while others claimed records had been falsified for years.

Last week, three people were arrested at the factory during a raid by police and trading standards officers.

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They were related to allegations of foreign meat being mislabelled as British, but the National Food Crime Unit of the Food Standards Agency’s (FSA) is looking into possible breaches of food hygiene rules.

Darren Davies, head of the unit, said: ‘We are carrying out a criminal investigation into how a supplier was allegedly providing products labelled as British when they were in fact sourced from elsewhere.

‘This is a complex and live investigation and we are looking into all new lines of enquiry with our partner organisations, including any potential food hygiene breaches at the premises.

‘If any evidence of a food safety risk is found, then necessary action will be taken.

‘The FSA advised retailers last year to check their cooked meat supply chain and to apply extra due diligence in their checks. We don’t give out these alerts without a reason.’

The FSA refused to name the supplier while it ‘painstakingly gathers evidence’ to support the investigation.

Speaking on behalf of the supermarkets, the British Retail Consortium confirmed they will support the FSA with its investigation into the supplier.

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Ex-workers described the company’s operation as ‘one big fiddle’.