Asda staff have been hit by an unexpected tax bill after a “quirk” in the calendar that could leave its employees having to pay back at least £12m in tax.

The grocer, which employees 170,000 people in the UK, paid all its staff for an extra four weeks in the last tax year and staff have now received PAYE tax bills ranging from £72 to £160 because they all underpaid income tax last year, the Dundee Courier reports.

The amount would be £12m if each member of staff had to pay back only the minimum figure.

One Twitter user said “Asda HQ genuinely have messed up on this one” although staff will not be out of pocket as they were originally paid the monies that will now be paid back.

An Asda spokeswoman said: “Asda colleagues are paid every four weeks rather than every month. Due to a quirk in the calendar that only happens once every 20 years, our colleagues were paid 14 times between April 6, 2012 and April 5, 2013 (the 2012/13 tax year) rather than the usual 13 times.

“This situation arises every so often for all companies that operate a four-weekly pay cycle. We communicated this HMRC issue to our colleagues in May and again this month.”

She added: “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.”