Calls for a criminal investigation into Barclays' recently resigned CEO Bob Diamond and individuals involved in the London Interbank Offered Rate (Libor) fixing scandal may meet a wall as would "too difficult" to mount a criminal charge against them, a top corporate barrister has claimed.

"Despite the findings from the Financial Services Authority [FSA] and others, it is still very difficult to mount a successful criminal prosecution," said Owen Watkins, corporate barrister at Lewis Silkin, who used to work at the FSA and before that the Securities and Investments Board.

"The authorities already know who they are and if the SFO [Serious Fraud Office] did want to bring about criminal charges, it would be the traders at the coalface that would most likely be the targets. The Serious Fraud Office would need to consider whether the evidence would support a charge of fraud and/or conspiracy to defraud.

"However, what people have to remember is that the authorities would have considered the prospects of a successful criminal prosecution during the time of their lengthy investigation into the Libor affair and after 18 months, they obviously had decided not to proceed down this route - either because there was no evidence of criminality or because they took the view that the evidence available was insufficient to support a successful criminal prosecution.

Public and political pressure

"In a criminal case, the prosecution has to satisfy a higher burden of proof - beyond reasonable doubt. People may think this is easier with the addition of civil proceedings but this is not the case," he added.

Diamond has quit his job following mounting public and political pressure since regulators in the US and UK fined Barclays £290m ($450m) over attempts to rig a key interest rate known as Libor.

"It seems to me unlikely that Bob Diamond would face criminal proceedings unless there is a smoking gun piece of evidence, for example in the form of an email or record of a phone call or a testimony from the upper echelons of Barclays," said Watkins.

"There are really long odds on a successful criminal prosecution of the most senior individuals for events that happen at the coalface of operations - they may simply not be aware of what is going on. But in this case the regulators can take civil action against them, for a failure to have proper systems and controls in place, for example."

Diamond's departure followed the resignation of Barclays chairman Marcus Agius, who indicated he would remain in his role until the search for his successor was complete. "The buck stops with me and I must acknowledge responsibility by standing aside," Agius said.

Japan and Switzerland may follow

Despite the settlement with the FSA, the US Commodity Futures Trading Commission and the US Department of Justice fraud section, Barclays still faces fines from other jurisdictions, such as in Japan and Switzerland because those regulators have not finished their investigations.

Another 17 banks, including RBS, and one broker, are also under investigation.

The FSA added that there was "a number of other significant cross-border investigations".

"We have a number of investigations that are ongoing," said Tracey McDermott, director of enforcement at the FSA. "Obviously we need to look at each case on its own particular facts but the initial indications are that Barclays was not the only firm nvolved in this."

The heat was turned up by investigators and MPs after the SFO confirmed that it was looking into potential breaches of the Fraud Act and was "considering whether it is both appropriate and possible to bring criminal prosecutions". It will relay its decision in a month.

However, Watkins emphasised that focus on launching a criminal charge would not be as fruitful as critics might think.

"We should perhaps not be too hung up over whether people are being prosecuted under civil or criminal charges because other than the understandable desire from the public and politicians to see some form of punishment applied, such as seeing handcuffs put onto people, it is the result that matters," said Watkins.

Banned from banking and mortgage industries

"With civil proceedings, there is the prospect of the regulators imposing an unlimited fine and banning individuals from the industry. This is potentially a very severe penalty and removes the possibility of that person offending again, which is a desirable regulatory outcome. What you have to also realise is that if criminal proceedings do take place there are still exceptional hurdles such as a higher standard of proof required - beyond reasonable doubt - to bring a successful prosecution."

According to the FSA's annual report published this month, the number of people being banned from the banking and mortgage industries has fluctuated over the last few years, with a recent peak in year ending April 2011.

During that year, some 71 people were excluded from the industry - up from 56 the previous year.

In the year ending April 2012, 47 were banned. In 2003, the figure stood at four.

During the unravelling of the Libor fixing scandal, critics called for prosecutions under the Sarbanes Oxley Act 2002 for failure of corporate officers to certify financial reports.

Under it, Diamond and other executives could face a personal fine of up to $1m and/or imprisonment of up to 10 years. The act was ratified in the aftermath of the collapse of Enron and WorldCom.

Sarbanes Oxley only applies to US courts. Experts said that in the UK there should be enough redress available through the FSA through other proceedings.

"Although Sarbanes Oxley is not law in the UK, don't forget that the FSA can bring civil proceedings against directors and senior managers of investment firms," said Watkins. "And the FSA has been very keen to make it clear to senior managers that they are responsible for how the business is run and will be separately liable from the firm they are running. For example, in the Greenlight Capital case this year, the FSA fined both Greenlight and CEO Einhorn for breaching rules."