The Equality and Human Rights Commission (EHRC) said it has uncovered "widespread evidence" of physical and verbal abuse and lack of proper health and safety protection.

Furthermore, workers often have little knowledge of their rights said the commission.

Migrant workers are most affected, but British agency employees face similar mistreatment, with many people afraid to raise concerns because they fear they will be sacked.

The EHRC said its inquiry, launched in 2008, uncovered frequent breaches of the law and licensing standards in meat processing factories, some of which supplied the UK's biggest supermarkets, as well as at employment agencies.

The commission made a series of recommendations aimed at improving recruitment practices and working conditions, as well as helping employees raise concerns.

A third of permanent workers and two-thirds of agency workers in the industry are migrants while at one in six meat processing sites involved in the study, every agency worker hired in the past year was a migrant worker.

The commission said this is partly due to difficulties in recruiting British workers for physically-demanding, low-paid work.

One in five workers who gave evidence to the commission said they had been pushed, kicked or had things thrown at them by line managers.

Meanwhile, a third revealed they had experienced or witnessed verbal abuse, often on a daily basis.

Workers also claimed they had been refused permission to go to the toilet, and one in four told the commission that pregnant workers had been mistreated, including the instant dismissal of agency workers who announced they were having a baby.

The commission said it had also found examples of firms which treated all workers with respect and dignity.

Neil Kinghan, director general of the EHRC, 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."

He went on: "We recognise that some retailers and processing firms have taken steps to operate in a way which improves the treatment of workers in the sector.

"However, there is still a lot that they and others could do. If the situation does not improve over the next 12 months, the commission will consider using its regulatory powers to enforce change where necessary."

Jack Dromey, deputy general secretary of Unite, said supermarkets should "hang their heads in shame".

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