The Serious Fraud Office (SFO) has dropped its criminal investigation into the alleged manipulation of global foreign exchange markets.
Some of the world's biggest banks - HSBC, RBS, UBS, JP Morgan Chase, Citibank and Bank of America - were fined a collective £2.6 billion by British and American regulators over their involvement in the scandal. And last year US authorities fined Barclays, JP Morgan, Citibank, RBS and UBS for misconduct in foreign exchange trading.
In a statement, the SFO said: "This decision follows a thorough and independent investigation lasting over one and a half years and involving in excess of half a million documents."
The Financial Conduct Authority (FCA), which passed on information to the SFO prompting the investigation, refused to comment on the decision.
"Based on the information and material we have obtained, that there is insufficient evidence for a realistic prospect of conviction," the SFO said.
"Whilst there were reasonable grounds to suspect the commission of offences involving serious or complex fraud, a detailed review of the available evidence led us to the conclusion that the alleged conduct, even if proven and taken at its highest, would not meet the evidential test required to mount a prosecution for an offence contrary to English law."
Mark Taylor, Dean and Professor of Finance at Warwick Business School, said: "I'm not really that surprised. They needed cast iron evidence of collusion.
"Obviously it's not strong enough to test in court. I don't blame the SFO if they didn't have the evidence.
"I'm sure there are a few relieved traders out there, to be honest. People might say there is an issue in that people got away with it, but they are innocent until proven guilty."
In August last year, former City trader Tom Hayes was sentenced to 14 years in prison after being found guilty of rigging the global Libor rates, which are responsible for around £3 trillion worth of transactions.
Hayes was the first person to go on trial over the scandal. He was accused of being the 'ring leader' in the scandal, which saw traders use chat rooms to manipulate the Libor, sometimes in exchange for something as small as a Mars bar. However, in January six City traders were cleared of the charges against them. Following the conclusion of that trial, Hayes' legal team raised questions about his conviction - they argued that given his position as a the 'ring leader', the innocent verdicts given to the other six traders would render him innocent as well.