The examples are many and varied, but appalling all the same.

Pregnant women being forced to stand for long hours in factory production lines without breaks, or perform heavy lifting under threat of the sack; meat factory workers having frozen hamburgers "like stones" thrown at them by line managers; women with heavy periods being refused toilet breaks so that they bled on their clothes on the production lines; workers with bladder problems refused breaks so that they urinated on themselves, workers exposed to verbal and physical abuse.

The "widespread mistreatment and exploitation" of agency workers, particularly migrant and pregnant workers, in meat and poultry processing factories, revealed in a report by the Equality and Human Rights Commission (EHRC) could be describing conditions in developing countries. But the inquiry, published today, focuses on employees in the UK.

Part of the largest manufacturing sector in Britain, the meat and poultry industry is worth hundreds of millions of pounds and employs 88,800 people. A total of 80% of processed meat goes to supermarkets.

The 15-month inquiry into recruitment and employment in the sector found that 70% of agency staff, a third of the workforce, are migrants. They are mainly Polish, followed by Lithuanian, Latvian, Czech, Slovakian and Portuguese. Some of the mistreatment of agency workers was illegal, it said, while other examples were a clear affront to respect and dignity. The report concluded:

• A fifth of workers interviewed reported physical abuse, being pushed, kicked or having things thrown at them by line managers. A Polish man working in a meat factory, said: "The managers …they would pull our clothes … and shout. They [threw] hamburgers. They were so angry because we were new and couldn't do the job as fast as we were supposed to … those frozen hamburgers are like stones."

• A third of workers said they experienced, or witnessed abuse, which was "bullying, humiliating and abusive". Some female workers said that women were verbally abused by line managers more than men and there were instances of sexual harassment.

One Brazilian man working in a poultry factory in the east of England, said: "I'll never forget it ... I'm not a slave. I just can't speak English. He talked to me like he talked with an animal. It is so terrible ... sometimes I don't even sleep in the night. Because the next day, I need to go to there [to that] horrible place".

• A quarter of workers mentioned poor treatment of pregnant workers and women attributed miscarriages to conditions. There were reports of pregnant workers forced to continue work that posed risks, including heavy lifting, standing for long periods, under the threat of being sacked. Some workers were given no further work once managers learned they were pregnancy.

• Some, including pregnant women, those with heavy periods and people with bladder problems, had been prevented from going to the toilet by their line managers.

• Some workers told ECHR they worked every day of the week without days off. The maximum number of hours worked a week regularly was 90 hours, while some shifts lasted 16-18 hours with only a few hours rest in between shifts.

• Workers spoke of pains to their limbs and extreme fatigue, partly due to carrying out repetitive tasks on fast-moving production lines for extended periods. There were also reports of a lack of protective equipment – only three in five firms said businesses provides the agency with a health and safety risk assessment. A Polish woman, working in a meat processing factory, said: "Working at a line, its really hard work. And the boxes are about 16kg each, so they are quite heavy and my hands are swollen at the end of the day... There are days I cannot open the door or keep a glass in my hand. I can't feel anything."

• Some workers spoke of being forced to work while tired after a long shift, ill or pregnant, under threat of losing their job. There were reports of an agency entering homes and waking sleeping people to make them work on their day off.

• More than eight out of 10 of the 260 workers EHCR interviewed said agency workers were treated worse than directly employed workers in all aspects, from poorer pay to allocation of poor jobs, to being treated like "second-class citizens".

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

Despite finding the workplace distressing and degrading, nearly one-third of workers endured this treatment without complaint because of fears that their work would be terminated and their chances of securing stable, permanent employment harmed.

These workers also had little knowledge of their rights or how to make complaints.

However, the report also found examples of firms who managed or treated permanent and agency workers of all nationalities with respect. One employer, Bernard Matthews, was mentioned as an employer of choice for agency workers for this reason.

One of the major problems highlighted by the commission was the way that supermarkets, as the largest customers of British meat, ordered products. Some agencies thought that the downward price pressure exerted by supermarkets and the way they went about ordering products from suppliers brought about the conditions that supported unethical traders.

One HR manager in a poultry processing plant said: "If the supermarkets' ordering system was better you could say for the next 10 weeks we want that amount of staff and we could recruit them."

Firms also said their ability to offer permanent employment is affected by fluctuations at short notice in supermarket orders.

The commission's 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 Gangmasters' Licensing Agency to help safeguard the welfare of workers. It said it would review their responses over the next 12 months and will consider taking enforcement action.