The former chief executive of Barclays, John Varley, has been acquitted of charges of conspiracy to commit fraud.
The Court of Appeal declined an application by the Serious Fraud Office to overturn a decision by Mr Justice Robert Jay that there was insufficient evidence against Mr Varley.
However, the other three defendants, Roger Jenkins, Tom Kalaris and Richard Boath, will now face a retrial.
The three defendants deny any wrongdoing.
The SFO accuses them of secretly paying £322m to secure investments from Qatar during the financial crisis.
The funding allowed Barclays to avoid a UK government bailout in 2008.
The case against Mr Varley was the only attempt to prosecute a chief executive of a major bank following the financial crisis.
It was seen as an important case for the SFO which had been criticised for the failure of previous prosecutions, including the collapse last year of a case against three Tesco executives.