Details of offshore credit cards are being studied as part of a crackdown on tax evasion, a law firm has said.
Interest earned on offshore accounts must be declared
HM Revenue & Customs (HMRC) has told credit card companies to hand over account details, according to legal firm DLA Piper Rudnick Gray Cary.
HMRC declined to comment on whether it had approached the card companies.
However, in a statement HMRC said it was "increasingly focussed on tackling tax evasion by people using offshore debit and credit cards".
The move by HMRC has been initiated by its Offshore Fraud Project Group (OFPG), DLA Piper said. The group was set up in 2003 to investigate UK residents who were suspected of evading tax using accounts in offshore tax havens such as the Channel Islands and the Isle of Man.
It is not illegal for a UK-domiciled person to hold money in an offshore account, but the interest earned must be declared.
Aileen Barry, national tax investigations director at DLA Piper, said the OFPG had issued more than 10 "production orders" on financial institutions in the UK.
These orders require the firms to give details of UK resident individuals who use offshore credit or debit cards, or money held overseas, to settle their bills in the UK.
"The information provided by the financial institutions has given the OFPG evidence with which it will mount a vast number of investigations into people who may have failed to declare all their income for tax purposes," said Ms Barry.
"On the basis of the information that it has gathered, thousands of people will be investigated for suspected tax evasion.
"Of those individuals, a number, almost certainly, will be prosecuted."
In a statement, HMRC said it was "well aware" that offshore accounts were used to avoid paying UK taxes.
"HMRC is increasingly focussed on tackling tax evasion by people using offshore debit and credit cards and we are using a very wide range of information sources to solve the problem in order to create a fair and level playing field for all taxpayers," the statement said.
"This means is it is no longer possible to evade UK tax by simply salting away money in an offshore account.
"The best way forward for those with offshore accounts that they have not told us about is to get in touch with us as soon as possible."