HM Revenue and Customs (HMRC) officials are targeting some 6,000 UK-based Swiss bank account holders in a clampdown on customers who may have failed to report all their income and gains.
The department announced plans today to write to residents and organisations holding accounts with HSBC in Geneva following information it received last year under a tax treaty.
The information showed that more than 6,000 individuals, companies, trusts and other bodies held accounts and investments with HSBC Geneva.
The revenue and customs body said it has already begun criminal and serious fraud investigations into more than 500 people and organisations holding these accounts, and it will write to people who have not yet come forward to give them a "window of opportunity".
If they fail to do so, HMRC said it will launch an investigation into their affairs, which could include a criminal investigation or result in penalties of up to 200% in some cases.
Dave Hartnett, HMRC's permanent secretary for tax, said: "This is not an amnesty.
"There are no special rates of penalty or interest for those who come forward voluntarily. This is an opportunity for those who have made errors in past returns to correct them.
"The net is closing on offshore evaders. Don't wait for HMRC to contact you. Come forward to us and make a full disclosure."
The announcement follows the signing of a tax agreement between Switzerland and the UK earlier this month which is set to raise billions of pounds for the UK from 2013 onwards.
The work will be led by HMRC's new offshore co-ordination unit, which becomes fully operational next month.
David Gauke, Exchequer Secretary to the Treasury, said: "The Government has shown its commitment to closing the tax gap by making an additional £917 million available to HMRC to tackle evasion, avoidance and fraud.
"This will fund the new offshore co-ordination unit and its specialist teams, which will drive forward this work."