Update: Since this story was published, Marks and Spencer and Aldi have stopped taking fresh chicken from the 2 Sisters factory featured in this investigation. Read more here

You probably haven't heard of the 2 Sisters Food Group but, if you like chicken, the chances are you have eaten some of the meat it processes.

2 Sisters is the largest supplier in the UK of chickens to supermarkets but an investigation by ITV News and the Guardian has uncovered a series of potential breaches of food safety rules in one of the company’s factories.

Our undercover reporter got a job at "Site D" in the West Midlands, which processes fresh chicken for Tesco, Sainsbury's, Aldi, Lidl and Marks and Spencer. But he found evidence that suggests some of the chicken 2 Sisters produces from this factory may not be as fresh as the use by date suggests.

Our reporter filmed workers changing both the date of slaughter and the source codes on crates of chicken crowns in such a way that artificially extended the meat’s shelf-life and rendered it almost untraceable in the event of an outbreak of food poisoning.

On several occasions he witnessed workers returning chicken to the production line after it had fallen to the floor. This may have caused it to become contaminated.

He saw meat of different ages being mixed together and he filmed Tesco's "exclusive" Willow Farms range being topped up with drumsticks that were originally packaged for Lidl.

We have spoken to more than 20 workers, past and present, who have stated that such practices take place.

Some also claim that the chicken supermarkets reject is often repackaged at the factory and sent out again.

2 Sisters employs people to patrol the production line and enforce food safety rules but the quality assurance workers we spoke to told us that they are intimidated by production managers and fear being sent home if they intervene.

2 Sisters is the largest supplier in the UK of chickens to supermarkets. Credit: ITV News

A former employee told us they were treated like "the enemy" by production staff and that they felt their job was "pointless".

We shared our findings with Professor Chris Elliot, a food scientist at Queen's University Belfast, who led the government's inquiry into food safety following the horsemeat scandal in 2013.

"There are major concerns about food hygiene...I think [your] report absolutely calls out for a full investigation," he told us. "I think the Food Standards Agency will take this very seriously, they will inspect and look at the premises and see if there are grounds to close the facility down."

Professor Elliot has inspected many food businesses in the past four years and insists he's never seen such poor standards. More generally, he believes government spending cuts have damaged the effectiveness of local authority inspections.

"It is my view that the environmental health services in the UK have been cut to the bone and cut to such an extent that things like this are probably happening more frequently than they should," he said.

Our reporter uncovered a series of potential breaches of food safety rules. Credit: ITV News/the Guardian

Dr Richard Hyde, an expert in food law at the University of Nottingham, told us: "If you are placing a use by date that is incorrect, that is a breach of law. If you place food on the market that doesn't have the correct traceability information, that is a criminal offence.

"There are a basket of potential offences here that the regulators need to look at and decide whether further action is required".

2 Sisters Food Group was founded in West Bromwich in 1993 by Ranjit Singh Boparan, who has built the business into one of the UK's largest food producers, employing 23,000 staff.

The 2 Sisters empire is vast but Ranjit Singh Boparan is still known in the West Midlands as "The Chicken King" and the bulk of the group’s sales come from processing poultry.

Boparan and his wife, Baljinder, own the business and The Sunday Times estimates their wealth to be £544 million.

Experts were shocked by the footage we showed them. Credit: ITV News

2 Sisters is dependent on the five biggest supermarkets for its income. The group’s accounts note that the loss of any of these “key customers” as a potential risk to the business. The same section notes that “product quality and safety issues are paramount to the groups’ success”.

2 Sisters Food Group told us that it takes these allegations extremely seriously. It said: “Hygiene and food safety will always be the number one priority within the business, and they remain at its very core.

"We also successfully operate in one of the most tightly-controlled and highly regulated food sectors in the world.

"We are subject to multiple and frequent unannounced audits from the FSA, BRC, Red Tractor, independent auditors as well as our customers. By example, our facility in the West Midlands under investigation received nine audits (five unannounced) in the months of July and August alone.

Employees at the factory told us about what it is like working there. Credit: ITV News

"However, we are never complacent and remain committed to continually improving our processes and procedures. If, on presentation of further evidence, it comes to light any verifiable transgressions have been made at any of our sites, we will leave no stone unturned in investigating and remedying the situation immediately.”

We took our findings to the supermarkets supplied by 2 Sisters Food Group. They told us that they take hygiene and traceability very serious. All said they would be investigating the allegations.

The responses from supermarkets to our investigation

There is no suggestion the supermarkets were aware of these practices.

They told us:

We take hygiene and traceability very seriously and have extremely high production standards. We are now looking into these allegations with our supplier. Marks & Spencer

We operate to the highest possible food quality and safety standards, carrying out our own regular audits at all of our suppliers to ensure these standards are maintained. As such, we take these allegations extremely seriously and will be carrying out our own rigorous investigation. Tesco

Lidl UK takes the issue of food safety extremely seriously and, as such, we conduct independent audits to ensure that our high quality and safety standards are met. Therefore, we are very disappointed to learn of these allegations and will be urgently investigating this matter with the supplier. Lidl

All of our suppliers are expected to meet our high standards. We are concerned by these findings and are investigating. Sainsbury's

We require all suppliers to adhere to the highest possible food hygiene and traceability standards at all times. Aldi

The response from the FSA