Britain's Serious Fraud Office has confirmed that it has formally launched an investigation into the rigging of the Libor inter-bank lending rate.
The announcement comes in the wake of a rate-fixing scandal involving Barclays that led to the resignation of three of the bank's most senior executives in a matter of days, including chief executive Bob Diamond.
An investigation by US and British regulators into the manipulation of Libor resulted in a record £290 million fine for Barclays.
Regulators found that Barclays staff had tried to affect rates over a number of years, first for profit and then to reduce concerns about how much it was being affected by the financial crisis.
The SFO said a few days ago that it was considering whether a criminal prosecution was appropriate and possible, and said this could take a month.
The UK government agency is responsible for investigating allegations of serious and complex frauds. It considers whether to prosecute using a number of criteria, including whether it is a matter of public concern, and whether the value of any fraud is more than £1 million.
Regulators are continuing to look into possible rate manipulation at other banks, while the US Department of Justice is carrying out its own criminal investigations.
The daily Libor poll asks banks at what rate they think they will be able to borrow money from each other in 10 major currencies, Reuters reports.
The result is used to determine interest rates on trillions of dollars worth of contracts worldwide.
The authorities have been examining whether traders tried to influence the rate to profit on bets on the direction it would go.