The Edinburgh-based lender, which is 80% owned by taxpayers, is expected to be handed the penalty by the Financial Conduct Authority tomorrow, according to reports.
RBS facing the £15m penalty, although other reports suggested it would be higher.
The fine was reported to relate to inadequate advice given to consumers who took out mortgages with the bank.
RBS and the FCA both declined to comment.
The fine will represent the latest in a string of financial hits the bank has faced including fines and compensation pay-outs in the wake of a series of scandals.
These include £3.25 billion to cover payment protection insurance (PPI) mis-selling and £1.3 billion for interest rate swaps - complex financial products which were sold to small firms.
It has also faced hundreds of millions of pounds in fines as part of the Libor rate-rigging scandal and also paid out to settle sanctions-busting allegations with US authorities.
RBS made a loss of £8.2 billion last year which included making provisions for past scandals as well as the cost of setting up its internal "bad bank" to dispose of unwanted toxic assets.
But it swung to a pre-tax profit of £2.65 billion for the first half of this year on the back of the resurgent economy.