Borrowers were told in July this year that the banks had miscalculated up to 18,000 mortgages and borrowers needed to make up the shortfall with higher mortgage repayments.
The FOS confirmed that it had upheld some of the customer complaints it received, helping them to fend off demands to pay the arrears.
However, it refused to disclose the number of complaints it had got about mortgage underpayments for either Yorkshire or Clydesdale bank.
Jason Clarke, spokesman for the Yorkshire Bank said: “In general terms, only a small number of customers have been affected and the vast majority have accepted bringing mortgage payments back up to speed.
“Those with a shortfall can expect to pay less than £25 a month extra on their mortgage repayments.”
He added that £2 and £3 of this figure was due to the past underpayment, and the rest had been put in place to bring the monthly mortgage back on track.
For those having problems repaying the shortfall, Clarke said: “We have written to those borrowers to discuss ways of putting the payments back on course. We are being as flexible as possible to those borrowers and looking at different solutions to help them pay back what they owe.”
Last week the FOS published its quarterly data on the number of complaints it received concerning each financial institution. It revealed that 4% of overall complaints were mortgage related in the third quarter.
