By Ian Pollock
BBC News personal finance reporter in Guernsey
Barclays bank offices, St. Peter Port, Guernsey
"Should five per cent appear too small, be thankful I don't take it all."
So sang The Beatles in their song Taxman, disgruntled at the 95% top rate of income tax they had to pay back in the 1960s.
Now, more than 110,000 UK residents who bank with Barclays are going to get a similar feeling.
Next month the Revenue & Customs (HMRC) will be given details of the bank's offshore accounts in places like the Isle of Man, Jersey and Guernsey.
The Revenue & Customs estimates it will soon be able to scoop up £1.5bn ($2.84bn) in unpaid tax, interest and penalties from Barclays customers alone.
But it seems the penny has yet to drop with many of the account holders.
And the authorities in one offshore location, Guernsey, believe the Revenue & Customs is massively over-estimating just how much money has been left sitting untaxed in bank accounts on the island.
The Special Commissioners
HMRC can barely disguise its glee at the legal victory it won at a hearing before the Special Commissioners in February.
The Commissioner, John Avery Jones, decided the tax authorities could demand that Barclays hand over the names and addresses for the offshore accounts of UK customers, along with transaction details for six selected months going back over the last six years.
A Revenue spokesman admitted that similar approaches were now likely to be taken towards other big banks.
"We are quite likely to take this forward," he says.
"This is a very big project for us. There is a lot of tax at stake."
Last year, HMRC, prompted by the desire of Chancellor Gordon Brown to cut down on tax evasion, set up a special unit called the Special Civil Investigations Offshore Fraud Projects Group.
Its director presented evidence to the Special Commissioners that hundreds of millions of pounds of interest had been accruing on deposits, which the account holders had failed to declare on their tax returns.
The tax inspectors also suspect that large sums have been salted away over the years without being subject to any income tax in the first place.
They quoted one eye-popping case of £576,000 in undeclared business takings being hidden in one account.
The end of offshore confidentiality
Barclays still advertises confidentiality as being one of the main advantages of having an offshore account.
Barclay's offshore accounts are not so confidential anymore
That is about to evaporate.
When Barclays hands over the information, it will be parcelled out to local tax offices.
They will check if the people have been declaring interest on their offshore accounts, or if they can explain unusually large amounts of money that do not seem to tally with their previously declared earnings.
Neither the Commissioners, Barclays nor HMRC will reveal exactly how many individuals could be investigated.
But the adjudication said they came to more than the number of UK residents who pay tax on foreign earnings.
That suggests at least 110,458 Barclays customers alone.
The bank is trying to appear laid back about all this and has yet to write to its affected customers.
"This will not affect the vast majority of offshore account holders who are using them perfectly properly," says a spokeswoman, Emma Rees.
"We haven't had a huge number of panic calls. The vast majority are pretty relaxed about it."
Accountants are never slow to spot a new line of business and in the last few weeks they've been busily warning their customers to come clean if they have somehow managed to "forget" telling HMRC about their offshore savings.
Accountancy firms like PWC are urging clients to confess
Peter Silk, a chartered accountant and spokesman for the Institute of Chartered Accountants in England & Wales (ICAEW), believes these latest developments may come as a bit of a shock to some clients.
"People have always thought they were ring fenced. I'm sure Joe Public doesn't realise what is around the corner," he says.
Some of the money was going to come to the attention of HMRC anyway.
The European Savings Directive came into effect last year.
It means that the banking authorities in offshore locations will now have to either impose a "withholding" tax on the European holders of offshore accounts, or send details of the money back to those customers' domestic tax authorities.
The difference now is that HMRC will be able to trawl back through records for the last six years.
No change in Guernsey?
In Guernsey, the financial services industry appears to be regarding all this with equanimity.
Peter Niven, Guernsey Finance
The 51 banks registered there account for more than half the economic activity of the Island.
The fact that deposits from around the world currently total about £80bn - more than half from Switzerland - reveals just how successful the island has been in running itself as a low-tax, offshore banking centre.
But the idea - repeated on the front page of the Financial Times recently - that people can simply bowl up with bags of used notes and deposit large sums of money, no questions asked, is vigorously challenged by Peter Niven, head of the joint government and trade body Guernsey Finance:
"Quite frankly, I was outraged. This is a slur against off-shore centres across the board.
"Over the last 10-15 years, we have put in all the required laws that international standards ask us to put in, to ensure we are not an easy touch and cannot be used for money laundering purposes.
"If somebody appeared with a stash of cash, they would have the greatest difficulty in putting that into an account here."
Just like UK banks, those in Guernsey have to know their customers and report any suspiciously large transactions in case these involve attempts to launder money.
For these reasons he reckons that 90-99% of tax dodgers have long been weeded out of the local system.
A new Docklands - style financial centre being built in St. Peter Port
He points out that hardly any accounts seems to have left the island as a result of the European Savings Directive coming into force.
He is also doubtful there is as much uncollected tax lying around in the island's bank accounts as the HMRC seems to think.
"The Revenue may be a little ambitious in what they think they'll get. I don't think they'll get anything like that £1.5bn."