Barclays Bank in Guernsey, one target of the Revenue's scrutiny

The Offshore Disclosure Facility, announced in April, has raised �140m so far from about 20,000 taxpayers.

But HMRC expects to get hundreds of millions of pounds more before the 26 November deadline next week.

Under Revenue pressure, about 64,000 taxpayers confessed this year to hiding untaxed interest in offshore accounts.

Most of them now have a final week in which to pay up.

200,000 offshore accounts

But they were only a small proportion of the 200,000 or so offshore account holders who were first targeted.

It's not just the interest at stake, it's the principal sum in these accounts that may also be taxable in many circumstances Ronnie Ludwig, Saffery Champness Offshore banking in Guernsey

The remaining ones, who did not respond at all, now face investigation and possibly prosecution.

"For the worst offenders, the Revenue will mount criminal investigations or a serious tax investigation, which will begin quickly after the deadline expires," said Chris Oates of accountants Ernst & Young.

"In all cases where people have not come forward they should expect HMRC to seek much more punitive penalties than the 10% under the amnesty."

Fear

The Revenue's main inducement to cough up has been its offer to levy a fine on the miscreant taxpayers of just 10% of the unpaid tax, instead of the 30% to 100% fines they might otherwise have imposed.

However the campaign has not been a tax "amnesty" as the unpaid tax, plus rolled up interest, will still need to be paid.

Ronnie Ludwig at accountants Saffery Champness believes the response so far has been low as the inducement to confess has not been generous enough.

"People are afraid," he said.

"It's not just the interest at stake, it's the principal sum in these accounts that may also be taxable in many circumstances."

As well as the proceeds of crime, such as drug smuggling and fraud, money may have been deposited by otherwise honest people.

They could be individuals who let properties abroad or who may have run businesses abroad and may not have realised that their income and profits are still taxable in the UK.

New offer

The Revenue's campaign was aimed initially at the customers of the main high street banks.

They sent their customers warning letters last year telling them they had been forced to hand over previously private account details to HMRC.

With the first "offer" now coming to an end, the Revenue says it is considering making a second one.

This will be aimed at people who have failed to pay tax on money in offshore accounts run by about 150 other financial institutions such as building societies, stockbrokers and investment management firms.

"HMRC will offer these customers some form of disclosure arrangement, ensuring that they are treated fairly, with similar terms to those in the first group," said an HMRC spokeswoman.

"The arrangements will, again, not be an amnesty because tax, interest and penalties will all be payable," she said.