A newspaper reported specific allegations such as a factory floor flooded with the guts of chickens and carcasses coming into contact with workers' boots and then being returned to the production line. It said the allegations related to two of the largest UK poultry processors, 2 Sisters Food Group and Faccenda.
Both companies denied the allegations. The newspaper's report was based on undercover footage, photographic evidence and information from whistleblowers relating to industry hygiene standards to prevent the contamination of chicken with the campylobacter bug.
Tesco, Sainsbury's and Marks & Spencer confirmed they had launched investigations into their chicken sources over the last week.
The most recent figures from the Food Standards Agency (FSA) suggested 65 per cent of raw shop-bought chicken was contaminated with campylobacter, the most common identified cause of UK food poisoning.
In Scotland, a health alert has been issued after more than 1,000 people become ill with vomiting and diarrhoea in five weeks - up 27 per cent on the average figure for the same period over the past three years.