Royal Bank of Scotland has suspended a third senior City currency trader over a global investigation into allegations of exchange rate rigging.

Ian Drysdale had been put on leave earlier and has now been suspended four months after similar moves involving colleagues Julian Munson and Paul Nash.

It comes after the Edinburgh bank, which is part owned by the taxpayer, said it was reviewing rules on currency dealers trading with their own money.

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The inquiry into online communications between traders and allegations of manipulating benchmark currency rates known as "fixings" has seen more than 20 traders at many of the world's biggest banks put on leave, suspended or fired.

The Bank of England, the regulator, the Financial Conduct Authority (FCA) and the US Federal Reserve and Department of Justice are looking into the allegations of wrongdoing in the $5.3 trillion-a-day global FX market, the world's biggest market.

FCA chief executive Martin Wheatley said the allegations were "every bit as bad" as those made in the interest rate-rigging scandal centring on the London Interbank Offered Rate, or Libor, which had already resulted in banks paying $6bn in fines and settlements.

Benchmark currency fixings are a cornerstone of global financial markets, used to price trillions of dollars worth of investments and deals and relied upon by companies, investors and central banks.

RBS would not comment on the case. Mr Drysdale could not be reached.

Meanwhile, RBS has agreed to pay $275m (£164m) to resolve a US lawsuit accusing it of misleading investors in mortgage-backed securities, lawyers for the plaintiffs said yesterday.

It still must be approved, but is the third-largest settlement in a class action against banks involved in mortgages at the heart of the 2008 financial crisis.