Twelve 2 Sisters poultry plants are being probed by the Food Standards Agency (FSA) after an undercover investigation by ITV News and the Guardian.
2 Sisters is the largest supplier in the UK of chickens to supermarkets but secret filming uncovered a series of potential breaches of food safety rules at a site in the West Midlands.
Now 12 sites in England, Scotland and Wales are under investigation.
Marks and Spencer, Lidl, Aldi, Sainsbury's and Tesco suspended taking chicken from the plant over hygiene concerns.
A statement from the FSA read: "This inspection has not identified food safety issues on these premises, but it highlighted issues requiring management attention, for example in relation to some aspects of staff training and stock control.
"We are working with the major retailers supplied by 2 Sisters, as well as reviewing information from various other sources."
2 Sisters Food Group said in a statement: “We receive regular audits at all our English and Welsh sites by the FSA throughout the year, so of course we will welcome our FSA colleagues at any location they wish to inspect in the coming days.”
Heather Hancock, Chairman of the FSA, said: "Consumers deserve food they can trust, and can be reassured that we take allegations of poor practice very seriously.
"Although our initial inspection found no risk to public health, we are broadening our investigations until we are satisfied that this is truly the case."
In an interview with Dave Lewis, the Tesco Chief Executive, earlier this week, he said he believed hygiene issues were isolated to the West Midlands site. He said that Tesco continued to source chicken from other 2 Sisters factories.
Speaking to ITV News, he added: "We've investigated and audited all the other sites that we take chicken from as a precaution, based on what it is you have raised. So we are confident that all of chicken we are taking is absolutely to the right standard."
At the West Midlands site, the undercover reporter witnessed:
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, workers returning chicken to the production line after it had fallen to the floor. This may have caused it to become contaminated
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
2 Sisters Food Group told ITV News that hygiene and food safety is "the number one priority within the business".
A spokesperson added: "We also successfully operate in one of the most tightly-controlled and highly regulated food sectors in the world."
The company launched a "comprehensive and thorough" internal investigation into its West Midlands site after viewing the evidence collected by ITV News.