When meats of different ages were mixed together, the slaughter date of the newest batch rather than the oldest batch would be used, employees claimed.
Some workers also claimed the chicken that supermarkets reject is sometimes repackaged at the factory and sent out again.
Quality assurance workers told journalists they were intimidated by production managers and worry about being sent home if they try to enforce food hygiene rules.
2SFG said in a statement on Sunday: “We are shocked and distressed by the allegations and the footage which we saw for the first time on Thursday September 28th.
“Since the allegations were put to us by the Guardian/ITV, we have been working around the clock to get to the truth of the matter.
“We responded immediately by launching our own internal investigation at our West Bromwich plant and invited the Food Standards Agency (FSA) to independently review our standards.
“The FSA has been in daily attendance since the allegations were raised and confirmed that it has not identified any breaches.
“However, our internal investigation has shown some isolated instances of non-compliance with our own quality management systems.
“We have therefore decided to temporarily suspend operations at the site to allow us the time to retrain all colleagues including management in all food safety and quality management systems.
“All colleagues will remain on full pay and will attend site whilst training is undertaken.
“We will only recommence supply once we are satisfied that our colleagues have been appropriately retrained.
“We continue to work closely with the FSA and our customers throughout this period.
“We remain committed to ensuring that we operate to the highest standards of hygiene and food safety, and we act with honesty and integrity at all times.”
On Sunday, Tesco became the latest supermarket to say it had decided to suspend buying chicken from the company, following Marks & Spencer, Aldi and Lidl.
2SFG was founded in 1993 and now produces one third of all of the poultry products consumed in the UK, and had revenues of £3.1 billion in 2016.
Founder Ranjit Singh Boparan and his wife have built up a personal fortune of £544 million according to the Sunday Times.
The company now has brands such as Fox’s Biscuits, Goodfella’s Pizza and the Harry Ramsden fish and chip chain in its stable, but poultry is still the company’s strongest revenue stream.
It counts supermarkets such as Tesco, Sainsbury’s and Aldi as well as M&S and Lidl among its biggest clients, but now several have vowed to suspend their relationship with the firm until the investigation has concluded.
The Food Standards Agency (FSA) said that it had found no evidence of breaches during an inspection of the plant following the Guardian and ITV News’ allegations on Thursday.
But it said it was still reviewing evidence, saying: “If any incidences of non-compliance are found we will take prompt and proportionate action with the business concerned, working closely with the local authority.”
Meanwhile, Mr Boparan may be called to Westminster to face MPs over hygiene standards at his company, the Guardian reported.
The chairman of Parliament’s environment, food and rural affairs committee, Neil Parish, said he wanted a “short, sharp inquiry” into practices at 2SFG.