The workers, mostly from South Asia, have spoken to The Guardian anonymously about their payslips that showed fewer hours than they worked.
Payslips also revealed workers earned €665 a month, instead of the minimum wage for agricultural workers in the region which is €680 (£570) a month.
Three farms in the Odemira region of southern Portugal, who either overwork or underpay their workers supply their berries to UK supermarkets through Californian-based berry seller Driscoll’s.
Driscoll’s berries are marketed to UK retailers by Berry Gardens.
Management at one of the farms that underpaid workers said their berries were supplied to Tesco, M&S and Waitrose.
However, the conditions that the berry pickers endured break worker welfare standards of Tesco, M&S and Waitrose and their supplier Driscoll’s.
“We are committed to upholding human rights and require all our suppliers to comply with both national laws and the provisions of the ETI Base Code,” a Tesco spokespersons told Grocery Gazette.
“This include full payments of all due wages. We are urgently investigating these allegations with our supplier Berry Gardens and will take any action required to ensure our standards are met.”
According to the Ethical Trading Initiative (ETI), which Tesco, M&S and Waitrose are members of, workers should be paid wages and benefits that meet national legal standards or industry benchmark standards that are even higher.
“Worker welfare is incredibly important to us, and we are very concerned to hear of these allegations. We are investigating fully with our berry supplier and an independent audit has begun to look into the concerns raised,” a Waitrose spokesperson told Grocery Gazette.
“We will take whatever steps that we need to in order to ensure that workers within our supply chains are treated fairly.”
Berry Gardens chief executive officer Nick Allen added: “All Driscoll’s farms noted will have had independent annual ethical audits which cover all aspects of social practice, on top of the relevant government inspections which are an essential part of our due diligence processes.”
“We are currently working with all those parties involved to fully investigate and address the claims made.”
Grocery Gazette has contacted M&S for comment.