The Financial Conduct Authority (FCA) is expected to provide further details, including the level of the fine, at 0900 GMT on Wednesday, the sources said.
It would be the latest setback to hit Chief Executive Ross McEwan, whose efforts to turn around the bank which made an 8.2 billion pound ($13.6 billion) loss last year are being hampered by ongoing investigations into past misconduct.
RBS, which is 81-percent owned by the British government, is among several major banks assisting regulators around the world investigating allegations of collusion and price-rigging in the global currency market.
It has already been fined 390 million pounds for its role in fixing the benchmark London interbank offered rate (Libor) and has set aside 3.2 billion pounds to compensate customers mis-sold loan insurance.