Extra tax collected by HMRC from capital gains tax investigations jumped 46% in the last year to £266m, up from £182m the year before.
The increase came as HMRC stepped up scrutiny of capital tax planning, said Lubbock Fine Wealth Management.
It said HMRC has been targeting cryptocurrency investors and amateur day traders for CGT investigations.
The number of CGT investigations closed by HMRC rose 26% to 9,800 last year, up from 7,800 the year before. Underpaid tax discovered per investigation also rose from £23,333 to £27,142.
The figures came from FOI requests submitted by Lubbock Fine.
The firm said the tax authorities have also been looking to collect more CGT from where an individual transfers shares or other assets to a family member for capital tax planning purposes.
The transfers may crystalise a capital gain for the individual that is transferring their assets, it said, adding that HMRC suspects that many individuals are underestimating the size and quantity of their disposals in order to cut CGT.
Graham Caddock, director at Lubbock Fine, said: “With inheritance tax planning around business assets becoming more common, HMRC is doing everything it can to ensure it collects every penny it can from those trying to reduce their CGT and IHT bills.”
He warned that families are increasingly finding that passing on wealth is no longer a straightforward exercise. “HMRC is scrutinising these transactions far more aggressively – not just for unpaid tax but it is increasingly challenging how assets are valued in the first place.”
The rapid growth in crypto investing is creating another major new area of focus for HMRC’s CGT investigators, Mr Craddock said. “Many younger crypto investors do not realise crypto profits are taxable and there is a perception that some crypto investors are more opposed to paying tax than the average investor.”
HMRC has significantly expanded its efforts to identify crypto investors through its Cryptoassets Disclosure Facility, with individuals who fail to disclose gains facing substantial penalties and interest charges.
Mr Caddock said: “Cryptocurrencies were renowned for being the wild west of investing. For many crypto investors this categorisation has stuck and many underestimate how seriously HMRC treats undeclared gains.
“Even worse, some crypto investors think that gains made through digital assets somehow sit outside the normal tax rules, which is exactly why HMRC is targeting the sector so aggressively.”