Sandfields Farms is part of G's, a multinational produce company growing onions, celery, mushrooms and radishes. Its customers include Waitrose, Marks & Spencer and Sainsbury's as well as Tesco and Asda.

At least 25 other victims were compelled to make garden sheds and fences for a Kidderminster firm which supplied major retailers such as Homebase, Argos and Wickes.

All the supermarket chains and other companies involved say they did not know the workers were being exploited. Several said they only became aware of the mistreatment after being contacted by journalists. 

Homebase, Travis Perkins and Wickes told the Sunday Times they have now launched investigations. There is no suggestion the firms were actively complicit...

A spokesman for Tesco said: 'We have a zero tolerance approach to exploitation in our supply chain and have stringent standards to protect human rights and prevent exploitation. All our UK suppliers are required to undergo training to spot the signs of modern slavery and prevent it occurring and audit their labour providers to ensure they are GLAA compliant. Our supplier has notified us of this investigation.' 

Asda referred the Daily Mail to the British Retail Consortium, who issued a statement on behalf of retailers affected by the slavery investigation.

Helen Dickinson, the BRC's chief executive, said: 'The BRC and our members believe that any form of human trafficking or labour exploitation is completely unacceptable. Retailers have a protocol in place aimed at supporting victims and enabling perpetrators to be brought to justice.

'Retailers are leading efforts to tackle labour exploitation and are careful to work with suppliers who provide proper working conditions for their workers. 

'However, this issue demonstrates that much more needs to be done, which is why we continue to call on the Government to strengthen the Modern Slavery Act and for greater enforcement to prevent exploitation now and in the future.' ...

Derek Wilkinson, Managing Director at Sandfields Farm, said the firm had 'invested heavily' in 'ethical awareness training and monitoring', adding: 'We are committed to working to the highest ethical standards and only work with fully accredited labour providers who work to the highest industry standards. 

All our suppliers are regularly audited to ensure their systems comply with UK law, ethical guidelines and the licensing standards of the GLAA (Gangmasters and Labour Abuse Authority). We will thoroughly investigate any alleged potential breach of these exacting standards and will cooperate fully with the relevant authorities as appropriate.' 

He said the firm operates an anonymous hotline to report suspected abuse or exploitation of staff, and line leaders are trained to spot signs of exploitation...