Britain's Serious Fraud Office (SFO) said on Friday it would formally investigate a bank rate-fixing scandal which has led to the resignation of three top Barclays executives.
"The SFO director David Green QC has today decided formally to accept the Libor matter for investigation," the independent government department said in a statement.
Libor (London Interbank Offered Rate), the rate at which banks lend to one another, is a flagship London instrument used as an interest benchmark throughout the world.
It plays a key role in global markets, affecting what banks, businesses and individuals pay to borrow money.
Last week, Barclays was fined £290 million ($452 million, 360 million euros) by British and US regulators for the attempted rigging of Libor and Euribor, its eurozone equivalent.
The SFO's announcement came two days after Barclays' former chief executive Bob Diamond appeared before a parliamentary committee to defend the bank's response to revelations that traders tried to manipulate the rates.
He quit the top job on Tuesday over the controversy, which has also forced Barclays chief operating officer Jerry del Missier and chairman Marcus Agius to resign.
Investigations by regulators into the rate-rigging are believed to be continuing into any role played by some other top names in international banking.