The Serious Fraud Office has closed its investigation into the $5.3 trillion (£3.7 trillion) a day foreign exchange market after almost two years, having sifted through in excess of half a million documents.

The decision to close the costly probe - the bill for which is likely to have run into millions of pounds - comes despite the crime agency admitting it suspects offences to have taken place.

"There were reasonable grounds to suspect the commission of offences involving serious or complex fraud," the SFO admitted.

However the fraud agency found, despite a detailed review of the evidence, that it does not have enough to secure the likelihood of convictions.

British and international banks have paid in excess of $10bn of fines to a variety of regulators over foreign exchange rigging in recent years, which has seen dozens of bankers lose their jobs.