Workers in the meat and poultry processing industry are widely mistreated, claims an official report into the sector.
Migrant and agency workers are frequently exploited with little knowledge of their rights, according to the Equality and Human Rights Commission.
Employees reported physical and verbal abuse and a lack of proper health and safety protection.
The treatment of pregnant workers was a particular concern.
Many workers had little knowledge of their rights and feared raising concerns would lead to dismissal.
While migrant workers were most affected, British agency workers also faced similar mistreatment.
The inquiry uncovered frequent breaches of the law and licensing standards in processing factories – some of which supply Britain’s big supermarkets.
One third of the permanent workforce and over two-thirds of agency workers in the industry are migrant workers.
At one in six meat processing sites involved in the study, every single agency worker used in the past 12 months was a migrant worker.
This is in part due to difficulties in recruiting British workers to what is physically demanding, low paid work.
It may also be due to perceptions among employers and agencies that British workers are either unable or unwilling to work in the sector.
Commission director general Neil Kinghan said: “The Commission’s inquiry reveals widespread and significant ill-treatment in the industry.
We have heard stories of workers subjected to bullying, violence and being humiliated and degraded by being denied toilet breaks.
“Some workers feel they have little choice but to put up with these conditions out of economic necessity.
“Others lack the language skills to understand and assert their rights.”
But the inquiry also found examples of good practice with firms treating permanent and agency workers of all nationalities with respect.
These firms benefited as a result, by being able to attract and retain well motivated, loyal and increasingly skilled workers.
But Mr Kinghan said the inquiry’s overall findings clearly showed that ethical auditing systems used by supermarkets were not uncovering mistreatment.
“While most supermarkets are carrying out audits of their suppliers, our evidence shows that these audits are not safeguarding workers.”
Supermarkets clearly needed to take steps to improve audits.
Processing firms and agencies supplying workers also needed to pay more than lip service to ensuring that workers are properly treated.
“If the situation does not improve over the next 12 months, the Commission will consider using its regulatory powers to enforce change where necessary.”
The Association of Labour Providers, which represents agencies supplying workers, said it welcomed the commission’s report.
In a statement, it said: “The recommendations merit careful study by government, regulators, supermarkets, labour providers and labour users.”
The association said it was willing to discuss the issues with the other parties involved in the meat supply chain.
But it would be difficult to implement recommendations, such as paying workers for travelling time and engaging workers on contracts of employment.
Neither would be possible unless there was a commitment from retailers and labour users to meet such costs, the association said.
“Past experience suggests that this is unlikely,” it warned.