The public spending watchdog said it's chasing almost £2bn that's potentially owed in taxes by the UK's richest people.
The National Audit Office (NAO) said HMRC's specialist unit recovered £416m in 2015 from 6,500 "high net worth individuals" with wealth of more than £20m each. But it's still trying to track down an estimated £1.9bn.
Perhaps putting it in perspective, tax inspectors have as yet, pursued only one successful criminal prosecution, according to a National Audit Office report. Over the past five years, two individuals have been criminally investigated leading to a solitary prosecution, while another 70 were pursued through the civil courts. It's aiming to increase the number of prosecutions to 100 by 2020.
And putting the difficulty of the task into perspective, the report also notes that each of these 6,500 individuals had on average four serious tax issues looked at by the unit. Around 4,000 inquiries have been open for more than three years. Phew.
The £416m figure is in addition to tax the wealthy individuals voluntarily declare (that came to more than £4.3bn in 2014-15).
The £1.9bn of tax that is "at risk" of not being received, is an estimate and not all of it will be owed once each case has been examined in detail, the NAO said.
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As part of efforts to maximise the tax it does recover, each of the high net worth individuals is assigned a "customer relationship manager" by HMRC. They have the exciting responsibility of "building an understanding of each taxpayer's affairs and behaviours".
When officials open a formal inquiry into a case, they can take years to resolve.