Final warnings are going to be sent to about 5,000 suspected offshore tax dodgers by the HM Revenue and Customs.
5,000 recalcitrant tax payers are getting a last chance to pay
The letters are part of the Revenue's continuing campaign to get people to pay tax on income they have hidden in offshore accounts.
The campaign, called the Offshore Disclosure Facility (ODF), was first launched last year and led to 45,000 people paying an extra £400m.
But the Revenue suspects there is still much more tax to be recovered.
There are estimated to be tens of thousands of people who failed to confess when first contacted by HMRC last year and who thus failed to pay the tax, plus interest and also the 10% penalty, by the deadline of 26 November 2007.
The new letter is intended to prompt them to come forward before a full-scale enquiry is launched by their local tax office.
However they will now have to pay higher penalties than during the first phase of the campaign, of at least 30% of the value of the unpaid tax.
Serious cases of evasion may still lead to prosecution.
The letters, along with forms, help sheets, and work sheets, will go out from Monday 17th March and the recipients will have 30 days to reply.
"If you do not return it, we will take further action that may involve formal proceedings to recover the duties, interest and penalties," the letter warns.
Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants (ACCA), said anyone receiving the letter should co-operate.
"They are depending very largely on the willingness of offshore account holders to inform the taxman of their financial details.
"But if this is not done proactively, the taxman will find other ways and means, such as targeting banks and finance providers.
"They are intent on getting the information they want," he added.