Royal Bank of Scotland has withheld the bonuses of 18 staff members under investigation for possible roles in rigging the foreign exchange market, the British lender has said.

More than 50 former and current traders are under investigation in the probe, launched after the British government-controlled bank was fined for failing to stop traders manipulating currency markets.

The bank said six employees were already being disciplined and three of those had been suspended, pending the outcome of the investigations.

The inquiry is intended to "rebuild trust" in the bank following the scandal, which broke in 2012, said the bank's head of conduct and regulatory affairs, John Pain.

"We are undertaking a robust and thorough review into the actions of the traders that caused this wrongdoing and the management that oversaw it," Pain said.

"This is a complicated process but also an essential one in order to identify culpability and accountability for this unacceptable misconduct.

"To be clear, no further bonus payments will be made or unvested bonus awards released to those in scope of the review until it has concluded."

The bank is 80% owned by the British government after it received the world's biggest ever banking bailout in the financial crisis.

It is the parent company of Ulster Bank and Britain’s Natwest.

British and US regulators last year fined RBS £391m (€498m) for rigging the benchmark interbank lending rate Libor, and it has paid £325m to European regulators for similar allegations.