Her Majesty's Revenue and Customs (HMRC) has been asked to investigate alleged tax avoidance by Prince Charles's £700m hereditary estate.

The duchy of Cornwall last year provided Charles with an income of £18m and HMRC's anti-avoidance group is now being asked to examine its non-payment of corporation tax following a potentially significant court ruling on its legal status.

The issue has been raised by an accountant investigating the tax affairs of the duchy – an agricultural, commercial and residential landowner.

He has analyzed the impact of a judicial ruling handed down last year. Anti-monarchy campaigners claim it shows the duchy is running "a well-entrenched tax avoidance scheme".

The duchy insists it "is not subject to corporation tax as it is not a separate legal entity for tax purposes". But John Angel, principal judge at the information rights tribunal, ruled last December it was a separate legal body to the prince.

Accountants now believe the ruling could leave the duchy exposed to the 24% levy on profits other organizations must pay. Any change to its tax status could result in a cut to the prince's income.

Republic, the campaign for an elected head of state, has asked HMRC's anti-avoidance team to investigate whether the ruling means the duchy is now "using a highly questionable interpretation of its legal status as a means of avoiding corporation tax obligations".

A spokesman for HMRC said it would evaluate the information and "take appropriate action". There is no suggestion any law has been breached. Clarence House strongly denies claims of avoidance.