A STASH of financial data about Britons living abroad is set to be handed to Revenue & Customs this week as part of a global transparency drive sweeping through the world’s financial centres.

Thousands of expats with undeclared savings accounts are potentially at risk of penalties once the information is transferred to the country where they live in September, according to experts.

The information transfer is part of a global initiative that is aimed at flushing out tax evaders but might also pose a threat to people who had not intentionally dodged their tax obligations, advisers said.

Some British expatriates wrongly thought that if they complied with UK tax rules, they would not have to declare their income in the countries where they lived. In most countries, however, people are taxed on their worldwide income, gains and, in some cases, wealth.

Jason Porter, a director of Blevin Francks, a financial planning firm, said there was a particular problem with individual savings accounts (Isas), which were taxable abroad even though they were tax-exempt in Britain.

“A lot of people who leave the UK look at their Isa accounts and still think they are tax-efficient.”

Mr Porter said anyone with undisclosed assets should seek advice about coming forward voluntarily, which was likely to result in reduced penalties.

But he warned that some countries imposed heavy fines, citing the case of Spain where “horrific” penalties for non-declaration could exceed the value of the assets.

Banks, building societies, insurance companies and investment companies are required to provide HMRC with data about their overseas clients’ accounts — including balances — before Thursday.

Similar deadlines apply in other countries which are taking part in the first wave of data exchanges under the transparency initiative known as the Common Reporting Standard (CRS).

Information will flow in both directions and HMRC will receive potentially valuable information from states such as Luxembourg, Malta and Cyprus in September. It has already received financial data from the Crown Dependencies and Overseas Territories.