Princes Charles is facing calls from Cornwall MP Andrew George to "come clean" about the Duchy of Cornwall's tax arrangements.
The prince received a record £19m from the £762m estate last year, but the estate pays no corporation tax.
The call by Mr George, Lib Dem MP for West Cornwall, came as the Duchy prepares to appear before the Commons Public Accounts Committee (PAC).
The Duchy said it was private, not a firm, so does not pay corporation tax.
'Turn new leaf'
The Duchy of Cornwall is an estate of about 131,000 acres, mostly in the South West, which is run by and provides an income for Prince Charles.
Official accounts show the prince's total income last year was £20.2m, of which £19m was from the Duchy.
Mr George said: "To be fair, the Duchy doesn't appear to be engaged in the industrial-scale style tax avoidance of Starbucks/Google.
"So it would help itself if it came clean about what it pays in tax, which, in any case, is mostly on a voluntary basis, and those taxes, like capital gains and corporation tax, which it does not pay."
The prince's principal private secretary, William Nye, and the estate's finance director, Keith Willis, have been summoned before the PAC on 15 July.
A Duchy of Cornwall spokesperson said: "The Duchy is not a company and is not therefore liable to pay corporation tax and the prince is not entitled to receive any capital gains from the Duchy, and therefore does not pay capital gains tax.
"The prince pays income tax voluntarily on the surplus of the Duchy of Cornwall at the highest rate, which was 50% in 2012-13, resulting in a total of £4.4m (including an element of VAT)."