The bank said that complaints at RBS were down 26 percent in the first half compared with the same period the previous year, while complaints at its NatWest division fell by 24 percent over the same period.

"We're seeing an improvement in the number of complaints customers have to make to us, but the numbers are still too high and we're determined to tackle that," a bank spokesperson said.

(Reporting by Matt Scuffham; Editing by Simon Jessop)