Britain's Serious Fraud Office said today it was dropping a criminal investigation opened in July 2014 into alleged malpractice on the foreign exchange market because of a lack of evidence.
"The Director of the Serious Fraud Office has today closed the SFO's investigation into allegations of fraudulent conduct in the foreign exchange market," the SFO said in a statement.
"This decision follows a thorough and independent investigation lasting over one and a half years and involving in excess of half a million documents.
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"There is insufficient evidence for a realistic prospect of conviction," it said, adding however that at the time the probe was opened "there were reasonable grounds to suspect the commission of offences involving serious or complex fraud".
The SFO added it would continue to liaise with the US Department of Justice, which is running its own ongoing investigation.
London is the biggest hub in the world for foreign exchange trading.
Allegations about rigging currency markets followed a series of far-reaching international scandals over the Libor lending rate starting in 2012 which have hit many of the world's biggest banks.
Libor is estimated to underpin some USD 500 trillion of contracts including mortgages and consumer loans.
