Tesco says 140,000 workers are to be compensated after a payroll blunder meant they were paid below the National Living Wage.
The supermarket chain said the issue affected both current and former colleagues, with the majority due up to £40 each and some receiving more.
Tesco said the mistake, spotted during an internal review, would cost it £9.7m to rectify.
Its statement said: “The review, which was conducted during the implementation of a new payroll system, found the voluntary contributions made by some colleagues to benefits such as pensions, childcare vouchers and cycle to work schemes, led to errors that resulted in their pay after salary sacrifice not reaching National Living Wage levels.
“Tesco is in the process of contacting affected colleagues, and will ensure they all receive their reimbursement payments by the end of March.
“They will also be contacting former colleagues to arrange payments too.”
The chief executive of the company’s UK and Ireland division, Matt Davies, added: “As soon as our own review identified this issue we took immediate action to resolve it and establish which colleagues are affected.
“We’ve apologised to our colleagues and our priority now is to talk to them about how this affects them individually, and make any necessary payments.”