Individuals hiding income in offshore bank accounts could be forced to pay their tax bill after HM Revenue & Customs won a legal victory against four leading banks, reports have said.

Details of around 100,000 customers will be handed over to Revenue officials according to the Financial Times.

The newspaper reports that the banks involved are HSBC, HBOS, Royal Bank of Scotland and Lloyds TSB.

HMRC will investigate for evidence of UK-domiciled individuals who have not declared income on money kept in offshore centres such as the Channel Islands, although secret bank accounts in Switzerland and some other offshore jurisdictions are not expected to be effected.

It has been legal to hold money offshore since the relaxation of exchange controls in 1979 but it is illegal to conceal any interest earned from the taxman.

John Avery Jones, special commissioner, was reported as saying he had sided with the Revenue against the banks because 'in my view, the information that the Revenue has already obtained raises serious questions that merit investigation and cannot be investigated by any other means'.

The FT reported that £275m is expected to be recovered in unpaid tax. This is significantly less than the estimated £1½bn that was expected after a similar ruling against Barclays Bank last year.

HMRC has offered to reduce tax bills by 10% a limited period if individuals come forward of their own accord. Overall, account holders face a bill for up to 20 years of unpaid tax and interest.

It is proposed that individuals will be given six weeks to disclose the existence of an offshore account, and a further four months to make a complete disclosure and pay the tax bill.

Some have expressed doubt that account holders will be able to get adequate advice in such a short period.