Unite, the UK’s biggest union, is promising to stage a demonstration outside this year’s NFU conference, when Sir Stuart Rose, M&S chief executive addresses delegates tomorrow.

The union is calling on M&S and Sir Stuart to "practice what you preach", and deliver on commitments to operate an ethical supply chain.

A statement reads: “Whilst M&S claims to be one of the leading ethical supermarkets, Unite has evidence that there is widespread discrimination in the treatment of workers in the meat supply chain. A permanent two-tier workforce amongst many suppliers has been created, starting conflict between migrant and indigenous workers and helping cause community disharmony.”

Unite claims to have given its evidence to M&S and the other major retailers more than a year ago. “Despite talks and independent research which backed the workers' claims, M&S has failed to take effective action and insist that the meat supply industry stops exploiting its workforce,” said the statement. “Unite believes supermarkets should face up to their responsibilities to workers and communities. M&S should put its warm words into action, and start putting people first.”

Unite joint general secretary, Tony Woodley, said: "The supermarket's drive to keep costs down within the supply chain has created a permanent two-tier workforce where mainly migrant agency workers are on even lower conditions than directly employed workers.

“This world famous brand needs to face up to its responsibilities and practice what it preaches."