LONDON - Britain said on Thursday it had brought in the fraud squad to investigate possible crimes and would tighten laws over attempts to manipulate lending rates, a scandal that has engulfed Barclays and is expected to spread to other banks.

Shares in Barclays tumbled as much as 18% at one point by midday trade, wiping out £4,2bn from its share price - the biggest one-day fall since 2009, according to Reuters data. Shares were down 14,8% at 3.13pm GMT.

The bank agreed to pay a record $453m fine to US and British regulators for trying to manipulate the London Interbank Offered Rate (Libor) in 2005-08. It is the first bank to settle in a case that also includes most of the world's other largest financial institutions.

"This is a scandal. It's extremely serious. They've paid a very large fine and quite rightly, but frankly the Barclays management team have some big questions to answer," Prime Minister David Cameron told the BBC.

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"Who was responsible? Who was going to take responsibility? How are they being held accountable?" he said.

Mr Cameron and Finance Minister George Osborne both said regulations would be reviewed and tightened if necessary.

Barclays CEO Bob Diamond has acknowledged that the settlement would damage customer trust in the bank, and said he and other senior executives would forgo bonuses this year.

Politicians, including some from Mr Cameron's Liberal Democrat coalitions partners, say Mr Diamond should resign.

The Libor rate, compiled from rates that banks pay each other for loans, is used throughout the financial system to set loan rates around the world.

The investigation - which disclosed e-mails in which bankers appeared to promised bottles of champagne to thank each other for help in setting the rates - has added to a storm of anger against the financial industry.

"Done ... for you big boy," read one message sent by a Barclays banker to one of the lender's traders, who had asked him to fix a key lending rate artificially low.

In another message, after Barclays submitted a rate that was lower than the previous day's, an external trader says: "Dude, I owe you big time! Come over one day after work and I'm opening a bottle of Bollinger."

Mr Osborne said the e-mail exchanges "read like an epitaph to an age of irresponsibility".

Laws brought in when the opposition Labour party was in power did not enable the Financial Services Authority regulator to impose criminal penalties for manipulating Libor, but the government would look at changing them, he said.

"As part of our review into Libor and the strength of the financial regulatory architecture, we will examine if there are any gaps in the criminal regime inherited by this government and we will take the necessary steps to address that," Mr Osborne said.

Police could still find ways to prosecute. A spokeswoman for the Serious Fraud Office confirmed the squad was in discussions with the Financial Services Authority regulator over the case.

The British Bankers' Association (BBA), which establishes the parameters for how Libor is set each day, said it was "shocked" by the behaviour that led to Wednesday's fine, and wanted the British government to look at imposing new rules.

The trade body announced a review of Libor in March after concerns spiralled during the credit crisis that banks were low-balling quotes for benchmarks used to price about $350-trillion in contracts ranging from corporate loans to home mortgages.

"The banks which contribute to the Libor rate must meet the necessary obligations to their regulators," the BBA said. "As part of this review we will now be asking the authorities to consider in what manner the Libor-setting mechanism should be regulated in the future."

The BBA has co-ordinated Libor since the 1980s in a process that is not officially regulated. The rates are compiled and distributed for the trade body each day by news and information provider Thomson Reuters, parent company of Reuters.

The figures are designed to reflect rates at which top banks believe they could borrow money from each other in 10 major currencies and for 15 borrowing periods, from overnight loans to 12 months. By manipulating them, banks could make their own balance sheets look better.

HEAD ABOVE THE PARAPET

One industry source familiar with the situation said it could take months for the next culprit to be nailed by regulators in the global rate fixing scandal.

More than 20 other banks are being probed by regulators from London to Japan and Brussels to North America. The source said Barclays stood out for the way it co-operated with regulators.

"Barclays was extremely co-operative and the majority of these cases come down to not just the complexity but also whether a firm is willing to co-operate and how much a firm is willing to co-operate above and beyond its legal requirements," the source said.

The spotlight is now swinging towards the likes of UBS, the Swiss bank that was among the first to suspend a clutch of traders and last July won partial immunity from some prosecutors in return for co-operation.

Others who have already fired, suspended or seen staff leave after internal investigations into attempted Libor manipulation include JPMorgan, Deutsche Bank, RBS and inter-dealer broker Icap.

"One of the reasons London is a major international financial centre is because of the perceived emphasis on trust and integrity in the London market," said Simon Culhane, CEO of the Chartered Institute for Securities and Investment. "This scandal can only serve to damage London's reputation."

Opposition Labour leader Ed Miliband - whose party was in power when the attempted manipulation occurred - said criminal prosecutions should follow the investigation.

"People struggling to make ends meet will be outraged and disgusted by the way bankers have been walking off with millions of pounds for rigging the market," he said.

Reuters