Royal Bank of Scotland has been fined £2.8 million for "multiple failings" in the way it handled customers' complaints, the City watchdog has said.
The Financial Services Authority said the part-nationalised bank responded inadequately to more than half of all the complaints it reviewed.
It said there was a "high risk" that customers of both RBS and its NatWest brand may not have been treated fairly due to a catalogue of failings in the bank's approach to handling routine complaints.
These failings included delays in responding to customers and poor quality investigations into complaints, with complaints handlers failing to obtain and take into account all relevant information when making a decision.
The group also issued correspondence which failed to fully address all of the concerns raised by customers, while it did not explain why complaints had been upheld or rejected.
Customers were also not given information on their right to refer their complaint to the Financial Ombudsman Service within the appropriate time period.
Overall, the FSA said 53% of the cases it reviewed showed deficient complaint handling, while in 62% of cases the bank failed to follow FSA rules on giving people information on the Financial Ombudsman Service and in 31% of cases it did not demonstrate fair outcomes for consumers.
The regulator said the problems stemmed from the fact that the bank did not give staff adequate training and guidance on how to properly handle a complaint.
It also failed to monitor complaint handling in branches and the management information produced was not enough to assess whether customers were being treated fairly. RBS also failed to ensure that complaint handlers properly reviewed complaints, taking into account all of the relevant factors.