Royal Bank of Scotland has frozen the bonuses of 18 traders as part of a continuing review of the alleged rigging of foreign exchange rates.

The bank, which remains 81% owned by taxpayers, said it had made the decision to hold the payments as it continues to investigate the conduct of more than 50 former and current employees.

Six employees are in what RBS described as a disciplinary process. Three of them have been suspended as the investigation continues.

John Pain, the head of conduct and regulatory affairs at the bank, said: “We are undertaking a robust and thorough review into the actions of the traders that caused this wrongdoing and the management that oversaw it. To be clear, no further bonus payments will be made or unvested bonus awards released until it has concluded.”

Last month, RBS was one of six major banks that paid UK and US authorities £2.6bn to settle allegations that their staff had rigged the currency market.

Two UK and US regulators said they had found a “free-for-all culture” rife on trading floors, which allowed the markets to be manipulated for five years, from January 2008 to October 2013.

At the time of the fines last month Martin Wheatley, chief executive the FCA, said the regulator would “not tolerate conduct which imperils market integrity or the wider UK financial system”.

About 30 bankers have been suspended across the industry since the penalties and last week the first was arrested by the Serious Fraud Office.