More than three million complaints were made by consumers to financial services firms in the second half of 2016, according to the City regulator.
Barclays Bank received the most complaints in the period, followed by Lloyds Bank and Bank of Scotland, which are both part of Lloyds Banking Group.
Barclays received 438,237 complaints while Lloyds Bank had 297,148 and Bank of Scotland 267,237.
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Although the overall total of 3.04 million was significantly higher than the 2.05 million reported in the first half of the year, the Financial Conduct Authority (FCA) said changes in the way firms report complaints were behind the increase.
Firms were not previously required to submit figures for complaints resolved by the end of the working day after they were received, but from 30 June all complaints became reportable.
Payment protection insurance (PPI) remained the most complained-about product in the second half followed by current accounts.
Some 60 per cent of complaints were upheld in favour of customers and total redress paid was £1.9 billion.
The top two reasons for complaints were admin and customer service together with the advising, selling and arranging of products.
Christopher Woolard, the FCA’s executive director of strategy and competition, said: “Consumers want a simple way to complain that does not leave them out of pocket. And they want to be assured that their concerns will be dealt with fairly and quickly.”