A global probe by US, UK and Swiss regulators is looking into whether banks colluded to set rates.
London-based executives at three other major banks have been put on leave, according to reports.
UBS, Barclays, Deutsche Bank and RBS have confirmed that they are working with the investigation.
The foreign exchange market is worth more than £3 trillion a day.
Similar to the Libor-fixing scandal which resulted in large fines for major banks last year, the current investigation is looking into allegations that traders used instant messaging to fix rates.
Foreign exchange rates are set at 4pm in London every day in a one-minute window, it is the conversations during this window that are being investigated.