Food and snacks eaten in some pubs have been prepared in "Dickensian sweatshop conditions" by migrant workers.
That is according to trade union Unite - formerly the Trade and General Workers Union.
The Union is concerned that young Polish workers employed by salad and vegetable preparation company Just Prepared in Bicester, Oxfordshire were forced to work extreme hours in an unsafe working environment.
The company has been trading for five months.
Just Prepared provides prepared vegetables and salads for an award-winning local company, which in turn supplies salad and vegetables to managed operator Mitchells & Butlers (M&B) and catering companies Brake Brothers and Rail Gourmet.
It is Dickensian, more in line with the nineteenth than the twenty-first century and another case of the sweatshop abuse of migrant workersAdrian JonesUnite
Unite says workers claim they were forced to work up to 16 hours a day in sodden clothes and could not access toilets without permission.
It also alleges that there were no contracts of employment or training.
Unite is urging Just Prepared in Bicester, Oxfordshire to work with it to improve conditions.
Dickensian conditions
"These workers have experienced a very distressing time at the hands of their employer, who made the most of their vulnerability and isolation to work them as hard as they possibly could without due regard for the workers' health or safety," said Unite regional industrial officer Adrian Jones.
"It is Dickensian, more in line with the nineteenth than the twenty-first century and another case of the sweatshop abuse of migrant workers."
M&B said it was carrying out a full investigation into the allegations made against Just Prepared. "Mitchells & Butlers takes the issues of employee health, safety and welfare within our supply chain very seriously and the company abides by the code of ethics issued by the Chartered Institute of Purchasing & Supply," said a spokeswoman.
"We would not work with suppliers who disregard any of these issues and we regularly audit our suppliers to ensure the required standards are consistently upheld."
Just Prepared
Just Prepared managing director Barry Spencer said: "Just Prepared Ltd takes its responsibilities as an employer very seriously.
"We go to great lengths to ensure our employees are well treated and work in safe, clean and pleasant conditions.
We go to great lengths to ensure our employees are well treated and work in safe, clean and pleasant conditionsBarry SpencerJust Prepared
"We comply with all relevant health and safety and employment laws, including the EU working time directive and national minimum wage legislation.
"Where they work additional hours, employees do this by choice because they want to boost their earnings.
"In that case they each sign a written opt-out agreement to that effect."
"We are more than happy to work with the union on resolving any employee related issues."