Yorkshire and Clydesdale Banks have admitted undercharging 18,000 customers on variable rate mortgages after a software error meant monthly payments were miscalculated.
The two banks, both owned by the National Australia Bank Group, expect customers to make up the shortfall on their mortgages, with some borrowers seeing monthly repayments soar by up to £300, The Herald reports.
Yorkshire and Clydesdale Banks insist the "vast majority" of its customers are unaffected and the error in its calculations has now been corrected.
Steve Reid, retail director for Clydesdale Bank, says: "First and foremost we are very sorry that this error has happened and for any inconvenience it may have caused those customers affected."
He adds of those affected, 99% have received a letter informing them of the impact to their mortgage account, while the remainder will hear from the banks in the next couple of weeks.
Yorkshire and Clydesdale Banks said customers who have been underpaying will see their mortgage payments increase to the correct amount with an additional amount to cover the shortfall.
They said for around half of their customers this would increase payments by less than £25 a month, of which £2 is to cover the shortfall.
It added if the suggested payment was "not suitable", they would provide other options and each customer's case would be reviewed individually.
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