People have until midnight to declare if they owe tax on money in offshore accounts - or they could face big fines and even court action.
HMRC wants account holders to own up to unpaid tax
HM Revenue & Customs (HMRC) has details on about 400,000 accounts held with the offshore branches of major banks.
If account holders come forward and tell HMRC they owe tax, they will be asked to repay back taxes, plus interest and a 10% fine.
HMRC have reported a last-minute rush of people trying to beat the deadline.
However, it is expected that only a minority of the 400,000 people whose accounts are known to HMRC will own up.
Last week, the BBC revealed that in a bid to reach more people, HMRC wrote to 200,000 of the 400,000 and urged them to come forward.
Some critics have said HMRC has been slow to publicise its "partial amnesty" offer, launched in April.
Failure to declare will lead to a wider investigation being launched and the prospect of 100% fines and even criminal prosecution.
Accountancy firms warn people who do not come forward that they are playing a high-risk game.
"Tens, possibly hundreds of thousands of taxpayers are hoping to be winners in a highly dangerous game of 'Russian roulette' with the taxman," said Reg Day, director of tax investigations at accountancy firm KPMG.
"They are gambling on the hope that HMRC will have so many cases to investigate that they won't get around to them.
"HMRC is a very patient beast and, although it may take some time, it will steadily work through the list," he added.
People who want to tell HMRC that they may owe tax on money held in offshore accounts can do so by logging onto the HMRC website.