Asda was approached by Unite which raised concerns about workers' rights
Thousands of temporary workers at firms supplying Asda will get pay rates equal to permanent workers after a deal was struck on working conditions.
The Unite union said that agency staff would be paid the same rate as other workers at Asda's 29 meat and poultry suppliers, which employ 6,000 people.
The union described it as a "groundbreaking" deal to improve conditions for agency workers.
New regulations on agency worker rights in the UK are planned for 2011.
There are an estimated 1.3 million agency workers in the UK and the government has been consulting on ways to offer greater working rights equality.
Unite has accused some supermarket chains of abusing their market power, leading to a two-tier labour market. Some agency workers are paid between 30p and £2 an hour less than permanent staff, Unite said.
A report by the Equality and Human Rights Commission will report in March on the meat industry in England and Wales.
As a result of the Asda deal, staff at its suppliers will be treated equally to permanent staff.
"We warmly welcome Asda's pioneering initiative which sends a clear message that one of Britain's biggest supermarkets is determined to put ethical principles into practice," said Unite's deputy general secretary, Jack Dromey.
"For years, supermarkets have driven down costs along their supply chain with tens of thousands of workers paying the price with discriminatory and unfair practices.
"It is wrong to exploit migrant agency workers on poorer conditions of employment and it is wrong to undercut directly employed workers on better conditions of employment. That divides workforces and damages social cohesion in local communities."
An Asda spokesman said: "We are committed to ensuring all the workers in our supply chain are treated fairly and ethically.
There are about 1.3 million agency workers in the UK
"Following our own investigation into working conditions in the meat sector, we agreed with Unite that agency workers who do the same work as permanent workers should receive equal pay, and that agency work should not be used as a means of preventing them from accessing the same rights as permanent workers.
"As a result we brought all our suppliers together as a group and presented both Unite's findings and the results of our own investigation. We, along with Unite, then met with individual suppliers to discuss the steps they could take to resolve these points of concern."
Other retailers will be considering the decision, ahead of new plans for agency workers which mean these workers will be given equal rights after 12 weeks.
Other benefits that will be offered from the first day of employment include information about job vacancies where they are working, equal access to facilities such as childcare and transport, and rights for working mothers, such as time to attend ante-natal appointments and parent-friendly working hours.
The move comes after the EU agreed the Agency Workers Directive at a meeting in Luxembourg in 2008.