More than three million complaints were received by firms in the second half of 2016, according to figures from the City regulator.
Some 3.04 million complaints were received in the last six months of the year - a steep increase on the 2.05 million reported in the first half.
The Financial Conduct Authority (FCA), which released the figures, said changes in the way firms report complaints were behind the jump.
Firms were not previously required to submit figures for complaints cleared up by the end of the working day after they were received, but from June 30 all complaints became reportable.
Payment protection insurance (PPI) remained the most complained-about product in the second half.
Around 895,000 PPI complaints were made in the last six months - accounting for 29% of reported complaints. In the first half, PPI accounted for 45% complaints under the previous rules.
Current accounts were the second most complained-about product, with 514,000 complaints making up 17% of the total.
Just over 172,000 complaints were made specifically about packaged bank accounts in the second half - making them the fifth most-complained about product.
Credit cards received around 313,000 complaints, and there were more 220,000 about motor and transport insurance.
Savings products including Isas generated more than 68,000 complaints.
The total redress paid to consumers was £1.9 billion, with 60% of complaints upheld in customers' favour.
Barclays Bank received the most complaints in the second half, 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.
The top two reasons for complaints, accounting for nearly four in five (80%), 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."