HMRC is about to start writing to UK residents and organisations with HSBC Swiss bank accounts who may not have reported all their income and gains in their tax returns.
The department is acting on information gleaned last year under a tax treaty, which revealed more than 6,000 individuals, companies and trusts held accounts and investments with HSBC Geneva.
Criminal and serious fraud investigations have already begun into 500 individuals and organisations holding these accounts, while many others have used HMRC’s Liechtenstein Disclosure Facility (LDF) to try and sort out their UK tax liabilities.
Those who have not yet come forward or are not currently under investigation will shortly receive a letter and will then have a window of opportunity to disclose all of their liabilities.
In instances where individuals do not come forward HMRC will begin an investigation which could result in penalties of up to 200%.
The departments new offshore coordination unit will conduct the investegations and will be fully operational as of 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 £917m available to HMRC to tackle evasion, avoidance and fraud.
"This will fund the new offshore coordination unit and its specialist teams, which will drive forward this work."
HMRC’s permanent secretary for tax, Dave Hartnett, was at pains to explain that this is not a tax 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 the past returns to correct them."
Under the current system, however, it is extraordinarily rare for individuals pay 100% of their tax liabilities.
So while HMRC is not creating any special rates or discounts, those who voluntarily come forward are likely to pay between 10-40%, depending on their circumstances.
For individuals to disclose under the LDF they would have to open an account in Liechtenstein and must fulfil certain criteria.
But even outside the LDF, historically, the HMRC will negotiate a percentage of the liability because they would rather receive some than none of the unpaid tax.
Simon Airey, partner at DLA Piper, who is familiar with the issue and been in discussions with HMRC, said: "The reality is HMRC are under pressure to collect revenue quickly and cannot staff 6,000 complex investigations at the same time. Those making a disclosure will pay a fraction of what they would have had to otherwise and will also receive a guarantee that they will not be prosecuted.
"Those who know that they have unpaid tax liabilities would be mad not to take advantage of this offer, but they would need to move quickly, the offer will only be open for acceptance for a short period of time. After this time HMRC will invoke the full civil investigation of fraud process or they will prosecute.”
The letter-sending follows the signing of a tax agreement between Switzerland and the UK earlier this month, which the government estimates will raise billions of pounds for the UK’s coffers from 2013 onwards.