Royal Bank of Scotland has suspended a senior currency trader in London, bringing to three the number of traders suspended by the bank since a global investigation into allegations of rigging reference exchange rates was launched last year.

Ian Drysdale was put on leave earlier this week and has now been suspended, a source familiar with the matter said.

This follows the suspension of Julian Munson and Paul Nash in October last year.

RBS declined to comment, and Drysdale could not be reached for comment.

On Tuesday RBS said it was reviewing rules on currency dealers trading with their own money.

The global probe 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, Britain's Financial Conduct Authority (FCA) and the U.S. Federal Reserve and Department of Justice are among those looking into the allegations of wrongdoing in the $5.3 trillion-a-day global FX market, the world's biggest market.

The FCA's chief executive Martin Wheatley has said the allegations are "every bit as bad" as those made in the interest rate-rigging scandal centering on the London Interbank Offered Rate, or Libor, which has already resulted in banks paying $6 billion 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.