Thousands of mortgage holders are facing huge rises in their monthly repayments thanks to a banking error.
Customers of the Yorkshire and Clydesdale Bank have been left with a shortfall on their variable rate mortgages after the bank miscalculated their monthly repayments.
The bank said on Wednesday that 18,000 customers had paid less than the contractual minimum monthly repayments needed to pay their mortgage within the agreed term.
It said historically low Bank of England interest base rates had exacerbated the problem.
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.
One mortgage payer, writing on a forum hosted by website moneysavingexpert.com, said she had been asked to pay a shortfall of more than £3,000.
Another customer, from Ayrshire in the west of Scotland, said: "I'm raging that I have never missed a single payment and have now been placed in over £2,000 of arrears through no fault of my own, effectively putting my home and family's security at risk."
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.