A trio of former City of London traders are to be charged by American prosecutors as part of a long-running probe into rigging the $5.3 trillion (NZ$7.5b) a day foreign exchange market.

It is understood the US Department of Justice is poised to bring criminal charges against the three - who were part of an online chat room called "The Cartel" which is alleged to have been used to manipulate foreign exchange - alleging they sought to manipulate the price of US dollars and euros in the currency market.

The three men are Chris Ashton, who was formerly global head of spot trading at Barclays, ex-JP Morgan dealer Richard Usher, and Rohan Ramchandani, formerly of Citigroup. Matt Gardiner, another member of "The Cartel" who used to work for Swiss bank UBS, has been helping the DoJ.

Given the trio are based in Britain, they will either have to go to the US voluntarily or American authorities will have to seek to extradite them.

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The charges are expected to be filed in New York.

Authorities around the world have been investigating a host of rigging scandals including the fixing of Libor, a key financial benchmark used to price trillions of pounds worth of securities, and manipulation of the currency markets.

In May 2015, Barclays, Citigroup, JP Morgan, and Royal Bank of Scotland pleaded guilty in the US to conspiring to rig currencies and agreed to over $2.5bn in fines in total.

The "Cartel" chatroom was at the centre of that case and prosecutors have said the now-infamous online forum was used between 2007 and 2013 to manipulate foreign exchange.

In the banks' sentencing last week, a Connecticut judge called on the authorities to take further action and said: "Frankly I would encourage the government to consider prosecution of individuals".

The DoJ is pushing ahead with the charges against Ashton, Usher and Ramchandani ahead of the inauguration on January 20 of incoming US president Donald Trump, who he is expected to shake-up the department.

The DoJ declined to comment. Lawyers for the men could not be reached.

It comes after former Barclays trader Jason Katz last week pleaded guilty to currency market price-fixing, the first individual to admit wrongdoing in the US criminal probe into foreign exchange.

In July last year, HSBC executive Mark Johnson was dramatically arrested at JFK Airport and, along with another executive who had by then left the bank, was charged over allegations of fraud linked to a $3.5b currency trade in 2011.

Johnson has pleaded not guilty.