The UK tax authority has overestimated how much money will be raised from an amnesty on undeclared offshore financial interests that closes on Friday, according to experts in the tax planning sector quoted by Reuters.

HM Revenue and Customs expects to raise up to £500 million ($749 million) over four years from tax paid on previously undeclared offshore assets as a result of the latest amnesty for which it is accepting online disclosures until 12 March. An initial disclosure deadline was in January and at that time, reports said that only a small number of 20,000 people with undeclared offshore accounts had come forward.

The amnesty – one of a number of such programmes operated by governments around the world – follows a similar process operated by the UK government around three years ago. Such amnesties meet with varied success: the first disclosure amnesty in 2007 for offshore accounts set up by the five largest high-street banks raised £400 million from around 45,000 people.

"To the best of my knowledge it has been incredibly poorly supported in terms of people coming forward," Ronnie Ludwig, a partner in the private client team at accountants Saffery Champness, was quoted as saying. Mr Ludwig, writing in WealthBriefing recently, has pointed out how governments have shown their ruthlessness in hunting alleged tax evaders by their use of stolen bank account data.

Mr Ludwig noted a reduced penalty of 10 per cent for people who come forward by the deadline, as opposed to potentially 100 per cent and possible criminal prosecution for those who do not and are caught, is not enough to convince many people.

"Other countries with amnesties... produced either no or low penalties," he said.

Other experts on such matters said the amount of money kept offshore for illegal purposes by UK taxpayers is small relative to other European countries.

"We just don't have that culture here that has made tax evasion endemic elsewhere in Europe," said Chris Groves, a partner at law firm Withers.

HMRC insists this will be the final opportunity to come clean, highlighting it has sought a tribunal order that means over 300 banks must submit details about accounts held offshore.