Complaints have flooded in to the Financial Ombudsman Service about Clydesdale Bank over the last six months after a botched systems tweak that saw the firm miscalculate mortgage repayments for thousands of customers.
The ombudsman received 410 mortgage-related complaints about the NAB subsidiary in the second half of 2010, up from just 66 in the first six month of the year.
The sharp rise is largely down to a calculation error caused by a systems tweak - revealed last summer - which resulted in around 18,000 variable rate mortgage customers underpaying. They are now being asked to make up the difference.
Across the industry as a whole, the ombudsman received 97,237 complaints during the past six months, up 15% on the first half of the year. The service upheld an average of 53% of complaints in favour of consumers, compared to 44% in the first half.
Lloyds Banking Group accounted for over 22,000 complaints, with Royal Bank of Scotland, Barclays and HSBC all racking up over 8000.
Natalie Ceeney, chief executive, Financial Ombudsman Service, says: "The latest set of complaints data continues to show that while some financial businesses are improving the way they handle their customers' complaints, some regrettably are not."