He will appear in federal court in New York on Wednesday and has been charged with conspiring to defraud a client of the bank.

Johnson joined HSBC in 2010 from a fund manager he had helped set up. He previously worked at Deutsche Bank.

He has been charged, along with former colleague Stuart Scott, with "front-running" a client order. Scott had been head of foreign-exchange cash trading for Europe, the Middle East, and Africa.

Front-running is where market makers trade on client information before the client's trade is executed. For example, if Trader A knows Client B is about to buy Currency C, then Trader A might buy Currency C before Client B's trade is processed. If the currency moves as a result of the client transaction, Trader A cashes in.