One in three super-rich are under investigation by HMRC over taxes

Just one high net worth person has been convicted in the past five years

Mr Bloomfield was on a list of HSBC clients with secret Swiss accounts. (PA)

One in three of the very richest people living in Britain are under “formal inquiry” at any one time by HMRC, official figures suggest.

A report by the National Audit Office (NAO) into the tax authority’s handling of “high net worth individuals” found that question marks over the financial affairs of such individuals were practically routine.

And, perhaps most scathingly, the report also revealed that the 380-strong taskforce set up up to catch the worst offenders has successfully prosecuted just one individual in five years.

The taxman is chasing a potential £1.9bn billion in taxes that have potentially gone unpaid by the super-rich, according to the NAO.

In the last five years, HMRC has also looked into and closed 72 cases into suspected tax fraud by high net worth individuals. Just one case led to a conviction.

HMRC opens formal inquiries when it does not agree with the position taken by a taxpayer or needs to clear up further information about their tax affairs. Around four issues are examined per high net worth taxpayer, the spending watchdog found.

The NAO said that though HMRC had “refined” its approach to collecting tax from the super rich – those worth over £10 million – in recent years, the tax authority should review its procedures with a view to making them more effective.

“HMRC is currently running a formal inquiry on around a third of high net worth taxpayers, with an average of four issues being examined per taxpayer,” the NAO report said.

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

“These... can take a long time to resolve with 6,000 issues... open for more than 18 months, 4,000 of which have been open for more than three years.”

There were around 6500 people classified as high net worth individuals by HMRC at the start of the current tax year; last year they paid £4.3 billion in tax.

The NAO said an increased focus on tax collection had paid dividends in recent years, with £416 million yield from enquiries by the “high net worth” unit in 2015-16 compared to £200 million in 2011-12 and an internal target of £250 million.

Amyas Morse, head of the National Audit Office, said today: “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.”

An HMRC spokesperson said: “Everyone must pay the tax that is due and we do not accept less.

"HMRC enforces the rules impartially and last year we tracked down an additional £416 million in tax from the wealthiest that would have otherwise gone unpaid. We will continue to evaluate our results so that we carry on getting what is due to the country.

"The NAO’s report recognises that taxing the wealthy is complex but also commends HMRC’s approach as both sensible and in line with OECD best practice advice.”