HMRC are planning a bulk mailing of letters in early November 2019 targeting wealthy UK offshore fund holders as the UK taxman launches a new probe of compliance and tax transparency.
The letters ask recipients to check that they have correctly declared money received from offshore collective investment funds, and include a factsheet that gives more details.
Chartered tax adviser at law firm Harbottle & Lewis Gary Ashford said: "It has long been the case that significant tax issues can arise from such investments.
It has long been the case that significant tax issues can arise from such investments"
"Given that the UK has just been through a campaign to report all offshore income and gains correctly or face 200% penalties I would urgently advise such investors to double check these matters have been correctly dealt with."
HMRC is chasing almost £2bn that is potentially owed in taxes by the UK's richest people, according to the public spending watchdog.
The National Audit Office said HMRC's specialist unit recovered £416m in 2015 from 6,500 "high net worth individuals" with wealth of more than £20m. But efforts are ongoing to recover an estimated £1.9bn, the NAO said.
Each one of the group of 6,500 is assigned their own HMRC official to liaise with over their tax bill.The £416m is in addition to tax the wealthy individuals voluntarily declare, which totalled more than £4.3bn in 2014-15.
They often have complex tax affairs involving different countries. The £1.9bn figure 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.
The sharp increase has been attributed to investigations following the Panama Papers leak, which alone yielded £190m, and new global data sharing agreements allowing the tax authority to identify cases more quickly.
Pinset Masons revealed that there are a further 215 similar investigations still ongoing.