Tax evaders who have hidden money and investments in Jersey, Guernsey or the Isle of Man are being given three and a half years -- until 30th September, 2016 -- to own up or face prosecution.
HM Revenue and Customs has announced details of the new voluntary "disclosure facilities", which were promised after the three Crown Dependencies agreed to start handing over the names of people with offshore accounts.
The Chancellor revealed in the budget that he hopes to realise £1bn in extra tax from the islands over a 5 year period.
Tax evaders manage to skirt around UK income tax and capital gains tax by parking their assets in overseas accounts and investment schemes.
Those who ignore the opportunity to come clean could face criminal prosecution, significantly bigger penalties, and having their names published.
Even people who own up in time could face penalties ranging from 10% to 40% of the sum owed, depending on the behaviour of the tax cheat.
Penalties for those who fail to come forward could rise to 100%, doubling the amount to be paid.
The new disclosure facilities come after a similar arrangement was drawn up to encourage tax evaders with accounts in Liechtenstein to come forward.