An error by the Yorkshire and Clydesdale Bank could see thousands of mortgage customers facing higher repayments each month.
The bank said some 18,000 customers have been left with a shortfall on their variable rate mortgages after it miscalculated their monthly repayments.
The problem had been exacerbated by the record low interest base rates, the bank said.
Yorkshire Bank is a division of Clydesdale Bank, which is in turn a subsidiary of National Australia Bank.
Affected customers, two thirds of whom are Clydesdale customers in Scotland, have now been asked to pay the correct monthly amount, plus an additional monthly sum to meet the shortfall.
While around half of the customers - some 9,000 people - have been asked to pay an increase of less than £25 a month, the rest face soaring repayments of up to £300 a month.
The bank said there were other options available to customers, including making a one-off payment to cover the shortfall, or extending their mortgage term, and they were dealing with problems on a case-by-case basis.
A bank spokesman added that the "vast majority" of mortgage customers were not affected by the issue, but added that there were around 180 customers who had not yet been told that they had underpaid.
