Barclays Group said that at 284,342 overall, its complaints for the first half of 2014 are down by more than one quarter (26%) compared with the first six months of 2013.
Within this figure, Barclays Group received 182,564 general insurance and PPI-related complaints in the first half of this year, showing a fall of more than one third (37%) on the same period a year earlier.
However, the group saw a 40% jump in mortgage-related complaints compared with the first half of 2013, with 5,900 complaints made in the first half of this year.
Barclays said the increase in mortgage complaints had been caused by a growth in its market share. It said the number of people taking out mortgages with it has increased by more than one third in the last year. It has put a review in place to work out the root causes of complaints and identified actions to improve customers' experiences.
The group also saw a 12% jump in banking complaints, with 91,198 recorded in the first half of this year. It said the upswing was largely driven by a restructure of the way overdraft charges are applied, which means some customers will now pay more to borrow, although the majority will pay the same or less.
RBS/NatWest said a "main driver" of the reduction in complaints it is seeing is due to a reduction in PPI complaints being brought by claims management companies.
To a lesser extent, the fall is due to a continued reduction on complaints arising from an IT glitch last December, it said.
RBS/NatWest said: "PPI continues to be a dominant factor in complaint volumes, but volumes are reducing."
Santander said it has seen complaints fall overall by almost one third (32%) year-on-year, with 135,611 complaints recorded in the first half of 2014. Some 30,234 of these complaints related to general insurance and PPI, marking a 50% fall on the first half of 2013.
Santander said it has introduced improved IT systems and continues to work on simplifying its product ranges.
Some £16 billion has been paid back to victims of the PPI mis-selling scandal since the start of 2011, making PPI the largest financial services redress exercise ever undertaken in the UK.
The figures were released as the Financial Conduct Authority (FCA) said that more than two and-a-half million old PPI cases are to be re-opened by banks, credit card providers and personal loan companies amid concerns that some consumers have paid too little compensation or have had their complaint unfairly rejected.
Financial services firms have a duty to report their complaints figures to the FCA, which publishes them each spring and autumn.