The number of people who admitted not paying tax on their overseas assets to the UK tax authority jumped by more than a third last year, prompted by warning letters sent by HM Revenue & Customs.

A freedom of information request revealed that 4,443 people confessed to not paying enough tax on their foreign assets to HM Revenue & Customs in 2021-22.

The figures were 35 per cent higher than the 2020-21 year, when 3,301 individuals admitted failing to meet their tax obligations on foreign assets. The increased disclosures boosted additional revenue from offshore income by 28 per cent from £44.5mn to £56.9mn over the period, the FOI showed.

Over the past year, HMRC has sent letters to people it suspects of not having paid the correct tax on foreign income and gains — including non-doms. The information is probably based on data it receives from other tax authorities.

Since 2014, international rules have led to the automatic exchange of information on financial accounts between tax authorities. The rules, developed by the OECD and known as the Common Reporting Standard, have been approved by 110 countries. They include historically popular tax havens such as Switzerland, Bermuda, the British Virgin Islands and the Cayman Islands.

“Hiding offshore income from the taxman gets harder each year. As more countries agree to share information about individuals, the chances of getting caught multiply,” said Sophie Warren, a tax investigations expert at law firm Pinsent Masons, which made the FOI.

“HMRC already has all the information it needs to prosecute thousands of people. These warning letters are really about giving people a final chance to come clean and receive a reduced penalty.”

Individuals can disclose unpaid tax on foreign assets using HMRC’s worldwide disclosure facility. The maximum penalty for failing to disclose offshore income is 200 per cent of tax owed. Failing to declare offshore income also carries a possible prison sentence of up to 12 months.

However, Warren said that provided an accurate and complete disclosure was made, the risk of criminal prosecution and penalties were “significantly reduced”.

HMRC said it “had a strong track record in tackling offshore non-compliance, whether stopping this through leading international activity, providing easy ways for people to correct their tax affairs or by acting strongly against those who are intent on breaking the law”.

Separately on Thursday, HMRC used its powers to name and shame two companies it said had facilitated tax avoidance to the public. It urged anyone using Absolute Outsourcing or Purple Pay Limited’s Equity Participation Scheme “to withdraw from them as soon as possible to prevent building up a large tax bill”.

Absolute Outsourcing said it was “disappointed that HMRC has published information connecting our business with tax avoidance”. As “an established provider of payroll services to the contractor industry”, it was dealing with the “inevitable worry” caused to clients and seeking to “engage with “HMRC to address and answer its claims”.

Purple Pay did not respond to a request for comment in time for publication.