Prime Minister David Cameron has said that Barclays Bank management has "serious questions" to answer over how it manipulated banking lending rates.

Barclays was fined £290m ($450m) after an investigation into claims that several banks manipulated the Libor rate at which they lend to each other.

The Chancellor George Osborne confirmed that HSBC, RBS, Citigroup and UBS are also under investigation.

The scandal hit Barclays shares, which ended Thursday trading down 15.5%.

Other banks shares fell as well, with RBS losing 11.5%, Lloyds down 3.9%, and HSBC giving up 2.6%.

Barclays had acknowledged on Wednesday that its actions between 2005 and 2009 had fallen "well short of standards".

Investigators say that Barclays' traders lied to make the bank look more secure during the financial crisis and, sometimes - working with traders at other banks - to make a profit.

'Who was responsible?'

Media playback is unsupported on your device Media caption David Cameron: "The whole senior management team have got some big questions to answer"

Mr Cameron said "the whole management team have got some serious questions to answer. Let them answer those questions first.

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

Business Secretary Vince Cable said that while it would be "seriously premature" to decide now whether Mr Diamond should be sacked, he added that the Government did have powers to disqualify directors.

"If the facts suggested action - and obviously we would be subject to legal advice; this is a legal process - then indeed that could well follow," he said.

In a statement to the House of Commons, Chancellor Osborne said: "The FSA report is a shocking indictment of the culture at banks like Barclays in the run-up to the financial crisis."

"Through 2005, 2006 and early 2007 we see evidence of systematic greed at the expense of financial integrity and stability and they knew what they were doing," he said.

Currently fines paid by banks help to bring down the levy other financial firms pay to run the regulator.

The Chancellor said the government is looking to change that to make sure the taxpayer benefits from the fines.

An influential shareholder tells me investors may demand the resignation of Marcus Agius as chairman of Barclays

Labour party leader, Ed Miliband, said: "This cannot be about a slap on the wrist.

"The people that have done the wrong thing in this case should face the full force of the law... including criminal prosecutions."

The Serious Fraud Office later confirmed it was in talks with the FSA about the case. This could result in UK criminal proceedings being brought.

The US Department of Justice had already confirmed that criminal investigations into "other financial institutions and individuals" were ongoing.

Regulators say that Barclays manipulated how it reported the interest rates at which banks lend to each other to benefit their traders and financial status.

Tracey McDermott, director of enforcement at the FSA, which imposed fines alongside the US financial regulator, told the BBC that "we need to look at each case on its own particular facts but the initial indications are that Barclays was not the only firm that was involved in this."

Media playback is unsupported on your device Media caption George Osborne: "How were such failures allowed to continue undetected and unchecked?"

The current chief executive of Barclays, Bob Diamond, was also in charge of the unit where the manipulations occurred, and is now facing calls for his resignation. On Wednesday, he and three of his top colleagues said they would give up their bonuses this year.

However the Liberal Democrat peer, Lord Oakeshott, said that if Mr Diamond had any shame, he would resign.

Martin Taylor, who was Barclays' chief executive from 1994 to 1998, said the bank had engaged in "systematic dishonesty".

"It's hard to believe that a policy which seems to be so systematic was not known by people at or very near the top of the bank."

Andrew Tyrie, chairman of the Commons Treasury Committee, said it would summon Mr Diamond to account for what had happened.

Mr Diamond later confirmed that he would appear before the committee.

Mortgage deals

Barclays' misconduct relates to the daily setting of the London Interbank Offered Rate (Libor) and the Euro Interbank Offered Rate (Euribor).

These are two of the most important interest rates in the global financial markets and directly influence the value of trillions of dollars of financial deals between banks and other institutions.

They can also affect lending rates to the public, for instance with some mortgage deals.

The British Bankers Association asked the government on Thursday to consider taking over the regulation of how Libor is set.

Crisis jargon buster Use the dropdown for easy-to-understand explanations of key financial terms: AAA-rating AAA-rating The best credit rating that can be given to a borrower's debts, indicating that the risk of borrowing defaulting is minuscule.

'Accepted culture'

The fine imposed on Barclays is part of an international investigation into the setting of interbank rates between 2005 and 2009.

Between 2005 and 2008, the Barclays staff who submitted estimates of their own interbank lending rates were frequently lobbied by its derivatives traders to put in figures which would benefit their trading positions, in order to produce a profit for the bank.

And between 2007 and 2009, during the height of the banking crisis, the staff put in artificially low figures, to avoid the suspicion that Barclays was under financial stress and thus having to borrow at noticeably higher rates than its competitors.

The FSA said Barclays traders were quite open about their routine attempts to manipulate rates.

"Requests to Barclays' submitters were made verbally and a large amount of email and instant message evidence consisting of derivatives traders' requests also exists," the FSA said.

In one instance, a trader recounted a conversation in which he had "begged" the submitter to put in a lower Libor figure.

"I'm like, dude, you're killing us," he said. His manager replied, "just tell him to... put it low".

In turn, the staff submitting the data would respond to the traders' requests.

"For you…anything," said one. "Done… for you big boy," said another.

And: "I owe you big time... I'm opening a bottle of Bollinger."