HM Revenue & Customs will shortly begin writing to UK residents and organisations holding Swiss bank accounts with the HSBC in Geneva who may not have reported all their income and gains to HMRC.

The department is acting on information received last year under a tax treaty. This revealed that more than 6,000 individuals, companies, trusts and other bodies held accounts and investments with HSBC Geneva.

HMRC has already begun criminal and serious fraud investigations into more than 500 individuals and organisations holding these accounts. To date, many others have taken advantage of HMRC’s Liechtenstein Disclosure Facility (LDF).

HMRC will shortly be writing to those who have not yet come forward, or are not currently under investigation. They will be offered a window of opportunity to contact HMRC and disclose all their tax liabilities.

If they do not come forward, HMRC says it will begin an investigation into their affairs, which could include a criminal investigation or result in penalties, in certain circumstances, of up to 200 per cent.

The work will be led by HMRC’s new Offshore Co-ordination Unit, which has recently been established and will become fully operational next month.

Exchequer Secretary to the Treasury, David Gauke, said: “The Government has shown its commitment to closing the tax gap by making an additional £917million 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.”

HMRC’s Permanent Secretary for Tax, Dave Hartnett, 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 will raise billions of pounds for the UK from 2013 onwards.