The Duchy of Cornwall's private estate has been likened to that of Starbucks, the company famed for using tax avoidance loopholes, by the chairwoman of the House of Commons public accounts committee.
Margaret Hodge said she believed no tax should be paid on a voluntary basis in an interview with Channel 4 Dispatches, as her committee prepares to scrutinise the Duchy's tax arrangements.
The Duchy of Cornwall makes only voluntary payments to HM Revenue & Customs and decides the sum it hands over.
Its income, from land, property and investments was up by 4% to a record £19 million during the last tax year, including £1.3 million in rent from Dartmoor prison and £2 million from Waitrose for use of a warehouse in Milton Keynes.
Ms Hodge, who is famed as an outspoken critic of tax avoidance, said "I never believe that any tax should be paid on a voluntary basis.
"Voluntary contribution [sic] from Starbucks may be welcome but they should be paying their fair share according to the profits they make in this country and I think the same goes for everybody.
"I think concerns are around transparency . . . and we [the committee] want to ensure that if it is a public institution it's managed in a way that is appropriate and that it is taxed in a way that is fair.
She added there was "a desire to have openness about what the Duchy of Cornwall as an entity owns, how it makes its money and how it pays it [and] whether or not it pays its fair share back into the common purse for the common good."
Prince Charles has paid voluntary income tax on any surplus from the Duchy estate since 1993.
He was also criticised recently by Labour peer Lord Berkeley for seemingly charging an above average rate for rent on its 700 tenancies in the Isles of Scilly.
Duchy tenants of the islands are charged an average of £130 a week, compared to £70 for council tenants and £100 for those in housing association accommodation.
A duchy spokesman said: "The Duchy of Cornwall is a private landed estate. It is not a public body, nor is it funded by the taxpayer . . . the Prince of Wales chooses to use his income from the duchy, rather than public money, to cover the great majority of the cost of the public duties of both himself and the Duchess of Cornwall, as well as the Duke and Duchess of Cambridge and Prince Harry. 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."
