RBS and NatWest have see a rise in complaints about the mis-selling of payment protection insurance (PPI).
In the first half of the year, PPI has made up 47% of complaints to RBS and 31% to NatWest.
They pushed up the total RBS complaints by 27% to 68,414 and NatWest rose 24% to 147,109.
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 admitted that complaints have risen to 202,384 in the first half of the year, of which PPI was the biggest part.
However, not all customer complaints will lead to a bank offering a compensation payment.
Customers who are turned down will have the right to take their complaints to the Financial Ombudsman Service (FOS), which has often found in favour of complainants.
The Financial Services Authority (FSA) revealed that the top 16 PPI sellers had paid compensation of £215million in the first half of the year, with nearly half of that being handed out in May and June alone.
All the big banks have admitted to huge potential bills for paying PPI compensation to their customers.