Top HSBC bankers were left quaking in their boots today when a whistleblower handed over details of more than 4,000 Britons with accounts with the bank in Jersey to the tax man.
HM Revenue and Customs are said to be rifling through the tax affairs of the offshore account holders but officially it said it was just looking over the information.
The accounts hold a total of £699 million - about £6.3 million for each of the 4,388 people on the list.
HSBC has already begun its own probe into how the information escaped its clutches.
The list is understood to include the names of serious criminals living in Britain, several celebrities and three bankers facing major fraud allegations.
The story came to light in a special investigation carried out by the Daily Telegraph.
It raises further questions about HSBC's procedures at a time when the group has revealed it is having to set aside yet more charges for money laundering breaches and mis-sold payment protection insurance.
It's looking at an extra £500 million to cover fines from US authorities which accused the bank of inadvertently allowing rogue states and drug cartels to launder billions of pounds through its US arm.
That brings the pot set aside to £935m million to cover the potential scandal but HSBC warned no agreement has been made and the cost could be "higher, possibly significantly higher."
Banks are required to report any suspicious activity that could suggest money laundering, although the responsibility for paying tax lies with the individual.
Banking union PCS said: "We welcome any moves that tackle tax avoidance, which deprives our public funds of much-needed revenue that is used for public services we all rely on."
And HSBC said it was looking into it but hadn't been officially notified of any investigation by HMRC or anyone else.
Foreign Minister Alistair Burt's admission that the Cameron government has "supported" a survey of attitudes to US drone strikes in Pakistan's tribal areas amounts to a tacit admission of British involvement.