Almost 4m complaints to firms in last six months of 2018 a fall of 5%
More than 3.9 million complaints were recorded by firms in the second half of 2018 - a fall of 5% compared with the previous six months.
The Financial Conduct Authority (FCA) said it is the first time complaints have fallen since 2016, when firms were required to change the way they report complaints.
Despite the overall downward trend, complaints about credit cards and motor and transport insurance have been going up.
Meanwhile, gripes about PPI and current accounts have decreased.
Complaints relating to TSB's IT problems in the first half of 2018 had previously helped to push current account complaints figures up, the FCA said.
The watchdog said there were 3.91 million complaints overall in the second half of 2018, down from 4.13 million in the first half of the year.
PPI remains the most complained-about product, accounting for four in 10 (40%) complaints. However, the number of PPI complaints decreased by 8% in the second half of the year, from 1.72 million in the first half of 2018 to 1.58 million.
The FCA said there was a change in reporting which allowed firms to exclude PPI complaints from the figures where it was established that the complainant had not purchased a PPI policy from the firm.
Current accounts remain the second most complained-about product, after PPI - although complaints about current accounts decreased by 13%.
By contrast, complaints about credit cards, the third most complained-about product, saw an increase of 10%.
The proportion of complaints about banking and credit card products which were upheld increased from 53% in the first half of 2018 to 56% in the second half.
And the number of motor and transport insurance complaints increased by 13%.
The proportion of complaints closed by firms within three days increased to 37%, compared with 35% in the first half of 2018.
Firms were also dealing with PPI complaints more quickly, with the proportion of these complaints closed within eight weeks increasing from 89% to 97%.
Christopher Woolard, executive director of strategy and competition at the FCA, said: "It is encouraging to see that complaint figures have dropped and firms are dealing with customer complaints more quickly. We expect firms to continue to focus on ensuring their customers are well-served and they respond quickly to complaints."
Here are the most complained-about products in the second half of 2018, with the number of complaints and the percentage change compared with the first half of 2018, according to the FCA:
1. Payment protection insurance (PPI), 1,581,849, minus 8%.
2. Current accounts, 604,894, minus 13%.
3. Credit cards, 386,195, plus 10%.
4. Motor and transport insurance, 283,664, plus 13%.
5. Other general insurance, 223,883, 0%.
Percentage of the 3.9 million complaints made which were in connection with PPI