HM Revenue and Customs (HMRC) sent more than 81,000 letters warning cryptocurrency holders they may owe capital gains tax in the past year.
A Freedom of Information (FOI) request seen by the BBC showed the number of letters sent by the tax authority had almost tripled since 2024.
Investors could face fines or prosecution if they fail to declare profits they made from selling, even if they exchange one cryptocurrency for another.
New powers due to be given to HMRC next year will make it easier to target wealthy crypto investors, with one analyst saying investigations will be like "shooting fish in a barrel".
"There is the expectation amongst tax authorities that cryptocurrency investment is rife with tax evasion," says Neela Chauhan, partner at UHY Hacker Young - which carried out the FOI.