HMRC targets HSBC Swiss accounts
The UK tax authority is targeting 6,000 UK-based Swiss bank account holders who may not have declared all their income and gains.
The names are understood to come from a disc originally stolen from HSBC's Geneva office.
HM Revenue and Customs (HMRC) has already started criminal and fraud investigations into more than 500 individuals and organisations.
Now it is writing to others telling them to declare their tax liabilities.
"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," said Dave Hartnett, HMRC's permanent secretary for tax.
"The net is closing on offshore evaders."
The information, which came through exchange powers with France, revealed the details of individuals, companies, trusts, and others who had accounts and investments with HSBC Geneva.
Those who failed to come forward could face an investigation that could lead to penalties of up to 200% of the unpaid tax.
Accountants are warning people to take the window of opportunity offered.
"There may be a legitimate reason why that person's name is on the data, for example, if they are just a signatory on the account, or if they are non-UK domiciled," said Fiona Fernie, tax investigations partner at BDO.
"Moreover, if account holders do not disclose and end up facing a tax fraud investigation, these are quite invasive - all tax affairs are scrutinized - so are a big concern for people."
The move is separate to a tax agreement signed between Switzerland and the UK in which undeclared money held by UK taxpayers in Swiss accounts will be taxed for the first time from 2013.
The UK authorities will also have the right to request the banking details of 500 UK individuals a year for further investigation.