Two more foreign exchange traders in London have been suspended by Royal Bank of Scotland on suspicion of market manipulation as part of an investigation into currency rigging by American regulators.

The Financial Conduct Authority is also escalating its supervision of forex traders in the City after the discovery of further misconduct by the US Department of Justice.

The latest cases relate to the alleged rigging of emerging market currencies, which did not form part of the FCA’s initial investigation.

Last year, six banks paid $4.3 billion in fines to settle with regulators from the United States, Britain and Switzerland. However, the inquiry was focused solely on the G10 currencies and reached back only to January 2008.

The American investigation is far broader, examining pre-crisis and emerging market currency trades. The City remains at the centre of the inquiry as it is the capital of the global $5.3 trillion-a-day forex market.

RBS’s action takes the total number of traders it has suspended over forex manipulation to five. In total, 35 traders have been suspended or dismissed in Britain since the scandal broke.

FCA insiders said that it would not be reopening its investigation and admitted that the banks that settled last year would not face any further fines even if more market abuse was uncovered.

Last year’s £1.1 billion deal between the FCA and five banks, including RBS, leaves Barclays exposed, however, as it chose not to settle. Sources said that Barclays could be treated differently by the FCA if both G10 and emerging market currency manipulation were uncovered, potentially exposing the bank to a bigger fine.

As part of last year’s deal, the FCA launched “an industry-wide super- visory programme” that required banks to “explore any read across into FX emerging markets, FX sales, derivatives and structured products referencing FX rates and precious metals”.

FCA insiders claimed that the regulator had “stepped up” its supervision into emerging market currency manipulation in recent weeks, but insisted that it had nothing to do with the US investigation. RBS is understood to have identified the traders as part of its own internal clean-up programme.

Signs that America’s regulators are again taking the lead in the crackdown on misconduct at British banks will deal a blow to the FCA. US regulators also took control in the Libor rate-rigging scandal, making Britain’s supervisors look lightweight when it came to industry supervision.

The US Department of Justice is continuing with its inquiry. In the UK, the Serious Fraud Office has a separate investigation ongoing.

An FCA spokesman said: “The supervisory measures we announced in November were about driving up standards across the industry. We will continue to work with firms to ensure this is the case across all of their trading business.”

The banking industry has faced billions of pounds of misconduct fines for rigging rate and currency markets as well as customer mis-selling since the financial crisis.