Hundreds of wealthy UK taxpayers have been sent letters by HM Revenue & Customs over possible large-scale tax evasion, the BBC has learned.
It is understood HMRC has acquired a list of high net-worth individuals with accounts at the Swiss division of HSBC.
The list was stolen by an employee and passed to the taxman by the French authorities. The bank is not accused of any wrongdoing.
The campaign comes after the government announced a crackdown on tax avoidance.
Chief Secretary to the Treasury Danny Alexander told the Liberal Democrat conference in Liverpool it was hoped that closing loopholes and ensuring wealthy people paid the full top rate of tax would generate an estimated £7bn a year by 2015 in additional income tax revenue.
Tax evasion and avoidance cost the Treasury an estimated £14bn a year and successive governments have vowed to take action against the problems.
BBC business correspondent Joe Lynam says the letters sent out by the HMRC are known as Code of Practice 9 and advise the recipients that they are suspected of committing illegal tax evasion which may lead to a criminal conviction.
A HMRC spokesman said: "The days of hiding money offshore to evade tax are now over."
Due to its secrecy laws, Switzerland has long attracted the very wealthy as a place to save their money.
This is changing in light of a worldwide clampdown on "offshore" tax havens ordered by the G20 last year.
It also follows similar efforts by Germany in 2008 against wealthy residents suspected of using banks in neighbouring Liechtenstein, another tax haven.
Germany's finance ministry paid an informant a reported 5 million euros (£4.2m) for a stolen computer disc containing the names of hundreds of clients at a wealth management firm.
In the HMRC case, a former staff member at HSBC's Swiss division stole highly sensitive data belonging to 15,000 high net-worth account holders earlier this year and fled to France.
The list was passed to the French authorities, who in turn handed the relevant details to HMRC.
HSBC fired the employee and the Swiss authorities are pursuing criminal action against him but cannot extradite him from France for legal reasons.
No more than 10% of the list of suspected tax evaders pertained to any one country.
HSBC said it had no comment to make on the matter.