A report from the National Audit Office showed that HMRC is “currently running a formal enquiry on around a third of high net worth individuals”.

HMRC is investigating risks from high net worth individuals with a potential value of £1.9 billion. This figure was an initial estimate of the tax that could be due at the start of 2015-16 and covers more than one tax year, the report said.

Of this, £1.1 billion relates to the use of marketed avoidance schemes; around 15% of high net worth individuals have used at least one scheme.

HMRC has identified that the risks from high net worth individuals relate “primarily to tax avoidance and the legal interpretation of complex tax issues, rather than tax evasion”.

Formal enquiries can take a long time to resolve, with 6,000 open for more than 18 months and 4,000 for more than three years. They are started when HMRC does not understand or agree with the position taken by a taxpayer and identifies a tax risk that cannot be easily resolved.

An HMRC specialist unit dedicated to collecting tax from high net worth individuals raised £416m from its compliance work with this group in 2015-16, according to the report.

This was separate from tax already voluntarily declared by these individuals.

At the start of 2015-16 HMRC considered there to be around 6,500 high net worth individuals, roughly 0.02% of all taxpayers.

In 2014-15, high net worth individuals paid over £4.3 billion in tax. This included £3.5 billion in income tax and national insurance (1.3% of the total revenue for those taxes) and £880 million in capital gains tax (15% of all CGT).

HMRC recorded yield of £416 million in 2015-16 from the work of the high net worth unit was an increase from £200 million in 2011-12, and exceeded HMRC’s internal target of £250 million in 2015-16.

Amyas Morse, head of the National Audit Office, said: "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. HMRC's specialist team gives it a better understanding of the tax affairs and behaviours of these taxpayers.

"While the yields from HMRC's work in this area have increased it needs to evaluate what approaches are the most effective and to understand the outcomes it achieves."