The BBC reports the government department will send 1,500 letters to people it believes have signed up to one particular avoidance scheme.
It is thought the action is the first of its kind and is seen as a “pre-emptive strike” before the scheme’s legal status is altered.
The BBC said about 700 of the letters would have arrived on Saturday. Four different versions of the letter have been issued.
One said: “”You are in the small minority of people who have made the deliberate choice to avoid tax. We focus our resources on this small minority. The choice that you have made changes the way we view your tax affairs.
“Our Specialist Investigations Unit will be carrying out a full investigation into this scheme and they will open an enquiry into your tax affairs.”
The National Audit Office (NAO) last week said tax avoidance schemes cost the UK more than £10bn. The report said HMRC was dealing with a backlog of about 41,000 aggressive tax avoidance cases.