THE REVENUE expects to claw back hundreds of millions of pounds from Britain’s wealthy, after banks handed over personal details of 400,000 customers with offshore accounts.

Revenue & Customs has uncovered City bonuses, inheritance windfalls and foreign holiday homes hidden in tax havens.

Dave Hartnett, the Revenue’s director-general, indicated he was “concerned” about the way in which offshore schemes had been sold by high street banks.

In an interview with The Sunday Times, he added that in the “majority” of cases, the offshore schemes may have been used illegally to avoid tax. In some cases, tax evasion continued for decades. He added: “We are talking hundreds of millions [in unpaid tax] here.”

Those with secret assets have been given a deadline of June 22 to make a declaration – or face criminal investigation into their tax affairs.

The level of offshore tax avoidance among Britons has never been quantified and the number of offshore accounts has surprised even tax professionals.

Over the past few months, high street banks – including Barclays, HSBC, and Lloyds TSB – have been forced to hand the Revenue details of their customers’ offshore accounts.

The names of hundreds of high-profile celebrities and businessmen are said to be among those implicated. A similar investigation into illicit offshore schemes in Ireland destroyed the careers of prominent politicians and businessmen.

Hartnett said the offensive could reap £1 billion, although some accountants believe the total tax raised could run to £5 billion.

The investigation may also spark a Financial Services Authority (FSA) probe into the high street banks’ activities.

Hartnett said: “Honest citizens have absolutely nothing to fear but those that should have made a disclosure but haven’t we will investigate. Quite often judges take the view that tax fraud is a crime against society. When there are decent sums of money involved you go to prison. Sad but true.”

In April, the Revenue announced a two-month “amnesty” for people to declare money hidden in offshore tax havens such as Jersey, the Isle of Man or the British Virgin Islands.

Hartnett said that the Revenue had so far been approached by 34,000 people and 4,000 have confirmed they would be disclosing hidden assets and paying tax they had dodged. He was expecting a flood of disclosures in the final week of the amnesty this month.

“People are coming forward, because, in my experience, people who have cheated the tax man worry about it,” he said.

“Over the last few years, the confidence that their offshore account would never be found has been eroded.”

He said that those caught up in the probe ranged from retired people evading small sums, to City bankers dodging six-figure tax bills.

“We had one guy who said he had been putting his City bonus in a Swiss bank account and from that account he and his wife had bought foreign property,” said Hartnett.

“They think their tax was wrong over a period of nine years and he has paid £200,000 so far.”

“A retired guy who got a lump sum on retirement decided to put it into a Channel Islands bank account and didn’t tell us about the interest. His tax liability was less than £10,000. . . It is just possible he could have got muddled up.”

“The biggest amount I have seen was more than half a million. But we think the serious disclosures will come at the end [of the amnesty]. We know of an accountancy firm that tells us they have 100 disclosures coming to us on behalf of their clients.”

Mike Warburton, a senior tax partner at Grant Thornton, said he had been approached by people who had been sold offshore tax schemes by banks but claimed to be unaware of the tax situation.

“People were told by bank staff that this was a great way of reducing tax, they didn’t realise they should actually be paying tax,” he said. “There appears to be a question as to whether aggressive salesmen at the banks were pushing these schemes.”

Hartnett echoed these concerns. “One of the things I am concerned about is the extent to which some of the marketing has . . . misled them,” he said.

“We will look at how offshore products have been marketed because part of our job is to try to make sure people understand their tax obligations and meet them, so if they are being misled we want to understand that. It would be an FSA matter and we have a statutory [obligation] to pass information to the FSA.”

The investigation marks a dramatic escalation in the Revenue’s attempts to target those with money offshore. In 2002, it ripped up a private agreement with Mohamed al-Fayed, prompting him to leave the country temporarily.

Most of the high street banks have set up separate offshore operations with branches in tax havens around the world.

The banks have written to their customers urging them to comply with the amnesty. Those coming forward must repay tax dating back up to 20 years but will only face a 10% flat penalty. Mohamed al-Fayed’s dealings with the Inland Revenue led him to leave Britain for Switzerland, a country whose tax regime has appealed to City high-flyers