Dutch NGO Somo said it had found evidence of abusive practices in Peru, such as "poverty wages" and discrimination against pregnant women, when it investigated three mango suppliers Sunshine Export SAC, Camposol AS and one other last year.
Somo claimed mangoes from two were sold in the UK variously by Tesco, Asda and Sainsbury's.
All three suppliers hit back at the allegations. The Ministry of Work had "concluded no infringement had been detected in Camposol with regards to labour," said a spokeswoman for the supplier. The company did not discriminate and complied with minimum legal remuneration, she added. Sunshine said it treated workers responsibly and provided more than the required minimum in terms of total remuneration.
A Sainsbury's spokeswoman said: "We source all our products from approved suppliers and expect them to meet the safety, quality and ethical standards we stipulate."
Asda said it worked closely with all suppliers to ensure workers were treated fairly, and that Sunshine and Camposol AS had both gone through independent ethical audits. "We found no evidence of worker mistreatment," said a spokesman.
Tesco said its suppliers were committed to meeting the Ethical Trading Initiative Base Code with producers.