HM Revenue and Customs (HMRC) has announced it is targeting 6,000 UK-based Swiss bank account holders in a bid to clampdown on those who may not have declared all their income.
The move comes after the Inland Revenue received information last year under a tax treaty, which revealed that more than 6,000 individuals, companies, trusts and other bodies held accounts and investments with HSBC Geneva.
HMRC has already started criminal and fraud investigations into more than 500 individuals and organisations and is now writing to others advising them to declare their tax liabilities.
The recipients of the request will be offered a window of opportunity to contact HMRC and disclose their financial dealings.
However, those that do not come forward could be subject to an investigation which may include penalties of up to 200 per cent of their liabilities.
The work will be conducted by the Revenue's new Offshore Co-ordination Unit, which will become fully operational next month.
HMRC's Permanent Secretary for Tax, Dave Hartnett, denied the move represented an amnesty on tax evaders.
"There are no special rates of penalty or interest for those who come forward voluntarily," he stated. "This is an opportunity for those who have made errors in past returns to correct them."
Mr Hartnett added: "The net is closing on offshore evaders. Don't wait for HMRC to contact you. Come forward to us and make a full disclosure."
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