New income tax codes for Welsh taxpayers have led to a mix-up in which some workers have wrongly paid Scottish rates.
A number of workers are thought to have paid either too much or too little income tax due to the confusion surrounding the codes, which are used to show where people live.
April saw the introduction of the Welsh Rates of Income tax, which carry the code C. However, HMRC said some employers had mistakenly entered S for Scotland, although it did not clarify how many workers were affected.
HMRC commented: "We have been made aware of an error in the application of new income tax codes for Welsh taxpayers by some employers, which has meant some taxpayers paid the incorrect amount of tax in April.”
Those earning between £12,501 and £14,549 would have underpaid by 1%, while income between £24,945 and £43,430 would have been taxed 1% more than the accurate figure.
Meanwhile, people earning from £43,431 to £50,000 would have seen a steep 21% hike on that portion of their income, and those who make more than £50,001 would have been overtaxed 1%.