HSBC's Swiss banking arm helped wealthy customers avoid tax and hide millions of dollars, according to a report by a network of investigative journalists.
The British banking giant provided accounts to international criminals, corrupt businessmen, politicians and celebrities, secret files analysed by the International Consortium of Investigative Journalists (ICIJ) show.
The documents have led to criminal investigations in several countries and attempts to get the money back after being stolen by an IT worker in 2007 and passed to authorities in France.
David Cameron has today been forced to defend Lord Green, who ran the bank during the period in question and was appointed as a trade minister in 2010 - the same year HM Revenue and Customs (HMRC) received data on potential British offenders in the files.
Details of the 30,000 accounts, which hold nearly £78bn of assets, are coming to light after the files were obtained by the French newspaper Le Monde and analysed by the ICIJ.
The files are reported to include evidence that the bank colluded with some clients to hide accounts from tax authorities in their home countries.
While holding a secret bank account is not illegal, they have been used by some to deliberately conceal assets to dodge tax, which is against the law.
"HSBC profited from doing business with arms dealers who channelled mortar bombs to child soldiers in Africa, bag men for Third World dictators, traffickers in blood diamonds and other international outlaws," the ICIJ reported.
According to the files, the bank's clients included former and current politicians from Britain, Russia, India and a number of African countries.
Those named in the files include people sanctioned by the US, such as Turkish businessman Selim Alguadis and Gennady Timchenko, an associate of Russian President Vladimir Putin who was the subject of sanctions over the Ukraine crisis.
HMRC was passed the data - known as the Lagarde List - in 2010 and has since then clawed back £135m from some of the 3,600 Britons identified as potentially avoiding tax.
But some MPs have complained about HMRC's perceived slow progress and the fact that only one evader has been prosecuted to date.
The revelations have sparked a blame game between the Conservatives and Labour over the failure to tackle the problem of hidden accounts and tax evasion.
Ed Miliband said there were questions over the appointment of Lord Green, a former chief executive and chairman at the bank.
He said: "I think this is a very serious situation and the Government has some serious questions to answer.
"We need to know why HMRC apparently did not act, apart from at the margins, on the information that they seem to have been given about what was going on.
"We need to know from the Government why they appointed Stephen Green of HSBC as a trade minister well after this information was passed to HMRC.
"I would like to see the Government explain what they did.
"We cannot have a country where tax avoidance is allowed to carry on and where government just turns a blind eye."
Commons Public Accounts Committee chair Margaret Hodge said she was "astonished" that Lord Green was not answering questions about the files.
She said the former HSBC chairman was either "complicit" or "wasn't on top of his job properly" and suggested he or current HSBC bosses could be called to face her committee on Wednesday.
The Prime Minister said Lord Green was an "excellent" trade minister who "did a good job".
City Minister David Gauke told Sky News he "was not aware of any evidence" that Lord Green had been involved in any improper activity.
Mr Gauke called on Shadow Chancellor Ed Balls, who was City Minister in the years up to 2007, to make an urgent statement in the Commons about what he knew about the claims and why the Labour government allowed large-scale tax avoidance and evasion to take place.
Responding to an urgent question from Labour in the Commons, Mr Gauke said the UK had been barred from seeking prosecutions on issues other than tax evasion, but French authorities have now agreed to provide "all assistance necessary" to allow HMRC to fully exploit the HSBC data.
Mr Balls said: "Nobody will fall for yet more desperate distraction tactics from George Osborne and the Tories when it is clear that this information was first given to the government in 2010."
The bank said in a statement that since the period in question, it had "implemented numerous initiatives designed to prevent its banking services being used to evade taxes or launder money".
"Although there are numerous legitimate reasons to have a Swiss bank account, in some cases individuals took advantage of bank secrecy to hold undeclared accounts," the statement continued.
"This resulted in private banks, including HSBC's Swiss private bank, having a number of clients that may not have fully met their applicable tax obligations.
"We have taken significant steps over the past several years to implement reforms and exit clients who did not meet strict new HSBC standards, including those where we had concerns in relation to tax compliance," it added.
"We are fully committed to the exchange of information with relevant authorities and are actively pursuing measures that ensure clients are tax transparent, even in advance of a regulatory or legal requirement to do so.
"We are also co-operating with relevant authorities investigating these matters and we acknowledge and are accountable for past control failures."
HMRC said in a statement: "We have systematically worked through all the Lagarde data.
"As a result tax, interest and penalties have now been paid by those who hid their assets in Switzerland to get out of paying tax.
"The decision to prosecute is made by the Crown Prosecution Service based on the facts."
