Complaints about the mis-selling of payment protection insurance (PPI) have surged at RBS and NatWest.
In the first half of the year, PPI gripes made up 31% of all NatWest complaints and 47% of those to RBS.
They pushed total NatWest complaints up 24% to 147,109 and RBS complaints up 27% to 68,414.
UK banks have until Thursday to tell customers how they are dealing with the PPI complaints that were on hold during a legal test case in the High Court.
Last week, Barclays revealed it had received 73,000 PPI complaints in the first half of the year and predicted they would keep on rising in the second half of 2011.
Lloyds also said recently that its overall insurance complaints had shot up to 202,384 in the first half of the year, of which PPI was the biggest part.
Not all complaints from customers will lead to a bank offering a compensation payment.
But customers who are turned down have the right to pursue their complaints to the Financial Ombudsman Service (FOS), which has often found in favour of complainants.
Thus the bill for the banks' mis-selling of PPI policies is rising rapidly.
On Tuesday, the Financial Services Authority (FSA) revealed that the top 16 PPI sellers had paid compensation of £215m in the first half of the year, with nearly half of that being handed out in May and June alone.
The recent deluge in payouts was triggered by the High Court's decision in April.
It slapped down a challenge by the banks to new FSA rules on how they should deal with old and new complaints about PPI.
This has led to a surge in the number of cases that now have to be processed after tens of thousands were put on hold by the banks pending the High Court ruling.
By contrast, the Santander banking group has reported a 14% drop in the total number of complaints it received in the first half of 2011.
They fell to 168,888 from 195,475 in the second half of 2010 .
The bank's business in the UK encompasses the former Abbey, Alliance & Leicester and Bradford & Bingley branches which it bought in a series of takeovers.
New complaints to Santander are now 31% lower than they were a year ago.
A bank spokesman explained that, unlike other banks, it had not been a party to the banking industry's legal action over PPI complaints, and had not put them on hold when they came in during the court case.
As a result, it has not had to deal with a big backlog in the past few months following the High Court's decision.
Most of Santander's new insurance complaints related to PPI and they fell from 18,588 in the second half of 2010 to 16,431 in the first half of this year.
All the big banks have admitted to huge potential bills for paying PPI compensation to their customers.
Lloyds has set aside £3.2bn, Barclays £1bn, RBS group £850m, Santander £731m and HSBC £269m.
At the start of August, the FOS revealed that it had been receiving 900 fresh PPI complaints every day and that these now made up 65% of its total workload.
It has found in favour of customers in 55% of cases.