Supermarket giant Tesco is to reimburse 140,000 current and former workers almost £10 million following an internal review of its reward systems.
Most of those affected will receive up to £40 each by the end of the month.
The review, held during the implementation of a new payroll system, found voluntary contributions made by some people 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.
Chief executive Matt Davies said: "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.
"We expect payments to be up to £40 for the majority of affected colleagues."
Usdaw national officer Pauline Foulkes said: "We continue to work with the company to ensure all staff affected are informed and that their pay is corrected and reimbursed as a matter of urgency.
"Our priority now is to agree measures with Tesco to ensure this doesn't happen again.
"In the meantime we are providing our members with the support and advice they require."
Business Minister Margot James said: "Today's announcement by Tesco shows our approach to the enforcement of the minimum wage is clearly working.
"We expose companies who fail to pay workers at least the minimum wage, but where there has been underpayment it is preferable that companies identify, and correct, that underpayment themselves.
"It is completely unacceptable for any worker to be underpaid and everybody should check their pay ahead of the national minimum and living wages rising on 1 April.
"Any worker who believes they may have been short-changed should contact Acas, the independent employment advice service."