One third of Britain's richest people are being investigated by HM Revenue & Customs as figures reveal that the taxman is chasing almost £2bn of unpaid taxes.

Around 2,160 British taxpayers are under investigation, each worth £20m or more, according to a report by the National Audit Office (NAO) which found that HMRC had collected £416m more than the super rich had originally declared.

The report estimates that £1.1bn is linked to aggressive tax avoidance schemes, which have been used by one in seven high net-worth individuals.

"Formal inquiries occur where HMRC does not understand or agree with the position taken by a taxpayer.

"These inquiries can take a long time to resolve with 6,000 issues under inquiry open for more than 18 months, 4,000 of which have been open for more than three years," the NAO said.

Over the last five years, HMRC has investigated and closed 72 cases of suspected tax fraud, although only two were criminally investigated and only one case led to a conviction.

The NAO said that HMRC needs to do more to to go after the super rich who do not pay their fair share.

"The tax affairs of the wealthiest in society are complex, making it harder for HMRC to ensure that they are paying the right amount of tax," Amyas Morse, head of the watchdog, said.

A specialist HMRC unit is investigating 40 wealthy individuals named in the Panama papers earlier this year, following the much-publicised leak at Panama-based law firm, Mossack Fonseca.

In the tax year 2014-15, the super-rich - which comprise of about 0.02 per cent of all taxpayers - paid more than £4.3bn in tax, down from £4.8bn in the tax year 2013-14.