The supermarket’s 170,000 employees were affected by the payroll anomaly that occurs for companies with four-weekly payroll systems, where 14 payments are made in a tax year rather than the usual 13.
Each of the staff owe HMRC between £72 and £160 in income tax for 2012/13 and if all of them owed the minimum amount, the bill would come to £12.24m.
Asda says it informed all its affected staff of the “week 53” issue in May and reminded them again this month.
An Asda spokesman said: “Due to a quirk in the calendar that only happens once every 20 years, our colleagues were paid 14 times between 6th April 2012 and 5th April 2013 - the 2012/13 tax year - rather than the usual 13 times.
“People service managers are on hand in our stores and head office sites to explain the process, but we encourage any colleague that is still unsure about their tax position to contact HMRC directly.”
Ceridian provides the payroll services for the supermarket and Simon Parsons, director of payments, benefits and compliance strategies at the payroll provider, said: “This is an anomaly within HMRC and there is no means of getting round it by payroll providers.”
Following the incident HMRC posted advice on its website on “dealing with ‘week 53’ payments”. When completing a Full Payment Submission it says: “Do not change the final tax code to week 1 if the only reason you have used week 1 is to calculate a payment on week 53.”