Regulators in the UK, US and Switzerland are looking into whether banks colluded to set exchange rates.
The global foreign exchange market is worth more than £3tn a day.
London is the most important centre for the market, accounting for about 40% of all foreign exchange trading.
The reports are that executives at Citigroup, JP Morgan and Standard Chartered have agreed to be placed on leave, but none has been accused of any wrongdoing.
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image: © Mark Ramsey