Thousands of workers in Britain's lucrative food industry are being subjected to widespread mistreatment and exploitation, including physical and verbal abuse and degrading working conditions, according to an inquiry published today.

The Equality and Human Rights Commission (EHRC) said it has uncovered significant evidence of abuse among producers supplying Britain's big supermarkets. The inquiry includes reports from meat factory workers who say they have had frozen burgers thrown at them by line managers, and accounts of pregnant women being forced to stand for long periods or perform heavy lifting under threat of the sack.

It also contained reports from women with heavy periods and people with bladder problems on production lines being denied toilet breaks and forced to endure the humiliation of bleeding and urinating on themselves.

One-fifth of workers interviewed, from across England and Wales, reported being pushed, kicked or having things thrown at them, while a third had experienced or witnessed verbal abuse.

The EHRC said some examples, such as forcing workers to do double shifts when ill or tired, were in breach of the law and licensing standards, while others were a "clear affront to respect and dignity".

Migrant workers are the most affected because one-third of permanent workers and two-thirds of agency workers in the industry are migrants, but British and other agency employees face similar ill-treatment, the report found. More than eight in 10 of the 260 workers who gave evidence to the commission said agency workers were treated worse than directly employed staff. The report found that 80% of processed meat goes to Britain's supermarkets, and that the main reason agency staff are used is to meet the big stores' demand.

Yesterday, Jack Dromey, the deputy general secretary of Unite union, which has campaigned for better rights for supermarket supply chain workers for four years, said: "Britain's supermarkets should hang their heads in shame."

Neil Kinghan, the EHRC director general, said: "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.

"While most supermarkets are carrying out audits of their suppliers, our evidence shows that these audits are not safeguarding workers and they clearly need to take steps to improve them. The processing firms themselves and the agencies supplying their workers also need to pay more than lip-service to ensuring that workers are not subjected to unlawful and unethical treatment."

Half of agencies and a third of processing firms said it was difficult to recruit British workers and that they thought they were deterred by the pay and conditions. A few British workers spoke of their difficulty registering for work with some agencies who supply almost exclusively Eastern European workers, which would be unlawful under the Race Relations Act.

Ian Livesay, the chief executive of the food picking and processing regulator, the Gangmasters Licensing Authority (GLA), said exploitation of workers was unacceptable and welcomed the report's recognition that the GLA has helped to improve standards in agencies and labour suppliers. He said: "We fully agree with the report's recommendation that supermarkets have a key role to play and, as the report recognises, we have signed an agreement with all the major retailers and their key suppliers to share information with us on serious breaches of our standards in their supply chain."

The government had recently increased its funding by £500,000 this year to cover enforcement and it now has 90 staff dedicated to stamping out abuse, he said.

Mark Boleat, chairman of the Association of Labour Providers, questioned the commission's methodology and suggested it had sought out workers who were experiencing abuse. He said: "How many workers did they interview? There are thousands in the meat industry. The workers are not a representative sample. I've never heard anything like that."

He said that some of the recommendations, such as paying workers for travelling time and engaging workers on contracts of employment rather than contracts for services, were impossible "unless there is a commitment from retailers and labour users to meet such costs, and past experience suggests that this is unlikely". If they were forced to offer contracts, many of its members would go bust, he said.

While it revealed many abuses, the EHRC report also highlighted examples of good practice, particularly when some firms did not differentiate between agency staff and directly employed workers.

The commission recommendations include: supermarkets improving their auditing of suppliers; processing firms and agencies improving recruitment practices, working environments and the ability of workers to raise issues of concern; and for the government to provide sufficient resources for the GLA to help safeguard the welfare and interests of workers.

Dromey said: "Supermarkets have driven down costs along their supply chain with tens of thousands of workers paying the price, suffering discrimination and unfair treatment.

"A two-tier labour market has been created, exploiting migrant agency workers on poorer conditions of employment and undercutting directly employed workers on better conditions of employment. That divides workforces and damages social cohesion in local communities. We welcome the call from the EHRC for workers doing the same job to be paid the same."