The government has obtained the largest ever disclosure order against Barclays bank, which will enable it to obtain details of hundreds of thousands of individual bank accounts to reclaim £1.5bn in tax.

The order, against the high street bank, will see offshore accounts scrutinised as part of what is thought to be one of the largest fraud campaigns ever in the UK.

Court papers show that HM Revenue & Customs expects to recover tax in 20% of the cases it is investigating, with a total yield of £1,508m.

The disclosure notice covers more people than fill in the foreign income sections of UK tax returns. HMRC had been alerted to the issues using information gained from debit cards and through tax credits paid to offshore accounts.

The notice requests information on non-UK bank accounts held by UK residents. Many, such as non-domiciliaries, may not have UK tax obligations on any interest earned on the accounts, but others will, giving rise to the notice.

An HMRC spokesman confirmed that the order was the largest ever in terms of the value of estimated tax it would recoup. A spokesman said: ‘Those who have been declaring the existence of offshore accounts as the law requires have absolutely nothing to worry about. The best advice to those that haven’t is to get in touch with HMRC as soon as possible.

‘The special commissioner’s ruling is a landmark decision. The action is about fairness and about creating a level playing field for all taxpayers.’

The disclosure notice is the work of the Offshore Fraud Project group, whose work in identifying and tackling fraud losses has so far hugely exceeded expectations.

Tom Rowbotham, director of tax risk at Deloitte, said he expected the notice to be the first in a series. ‘This represents a significant figure towards HMRC’s targets in reducing the tax gap,’ he said.