The Serious Fraud Office (SFO) has closed its investigation into allegations of fraudulent conduct on the foreign exchange market, raising questions over government decisions that have limited its powers.

Last week the SFO said that after reviewing 500,000 documents, it decided ‘there were reasonable grounds to suspect the commission of offences involving serious or complex fraud’, but that ‘based on the information and material we have obtained, there is insufficient evidence for a realistic prospect of conviction’.

A parallel investigation in the US remains live.

SFO director David Green QC had lobbied unsuccessfully for proposals that would have increased personal and corporate liability for financial crimes. Lawyers said the decision reopens the debate over whether government policy has emasculated the SFO in this area.

Sara Teasdale, a partner at criminal litigation law firm Byrne and Partners, said: ‘The SFO’s decision is all the more embarrassing when viewed in stark contrast to the US Department of Justice’s extraction of guilty pleas from Barclays, RBS, Citigroup and JP Morgan, and the swingeing fines imposed by the FCA and US regulators upon six banks, all in relation to the same factual matrix recently considered by the SFO.’