Prince Charles and the Queen were today under renewed pressure to open up their accounts to public scrutiny.

MPs on the powerful Public Accounts Committee have demanded to know how the businesses which provide his main income manage to avoid paying corporation or capital gains tax.

The Tory MP Edward Leigh, chairman of the PAC, has written to the Treasury questioning whether there was "anything about the status" of the Duchies of Cornwall and Lancaster that made them exempt.

The Duchy of Cornwall is Prince Charles’ main source of income, while the Duchy of Lancaster provides income for the Queen.

Experts have calculated the tax breaks could be worth up to £500,000 to Prince Charles each year with the cumulative saving over the years adding up to many millions.

Gordon Brown, the Chancellor, has backed Prince Charles in his refusal to open up the duchies of Cornwall and Lancaster to government auditors. Prince Charles has argued that there is no need for an audit of his figures as the books are already looked at by both PricewaterhouseCoopers and the Treasury.

Earlier this year the Prince of Wales attempted to placate MPs by revealing that he voluntarily paid £3.3m in tax on more than £14million income from the Duchy of Cornwall.

The letter from Mr Leigh, the second attempt by the PAC to force the Treasury to justify the Prince's tax situation, rejects the Treasury's attempt to shield Charles.

The missive, to Financial Secretary John Healey, states: "The Committee has considered the Treasury’s response on this point, but to help their understanding members would welcome a fuller explanation.

"In particular, the Committee would welcome the Treasury’s view on whether there is anything about the status of the Duchy of Cornwall and Duchy of Lancaster that puts them outside the regimes for corporation tax and capital gains tax.

"The Committee would also like to know whether the commissioners for HM Revenue and Customs have ever made a determination in relation to the Duchy of Cornwall or Duchy of Lancaster."

The Duchy of Cornwall was created in 1337 by Edward III for his son the Black Prince. Comprising 140,000 acres across 20 counties it provides income for heirs to the throne. The value of the estate grew by nearly £46m last year to £551m.

In further letters to the Duchies of Cornwall and Lancaster, Mr Leigh insisted that the PAC has a statutory duty to inspect the Duchies’ accounts.

He said there was a "paradoxical situation" in which the accounts were laid before Parliament but the Auditor General, Sir John Bourn, "lacks access to the underlying books and records".

A Clarence House spokeswoman said: "The PAC have asked again if the National Audit Office can audit the finances of the Duchy of Cornwall. They have asked previously.

"The Duchy is a private estate that provides an income for the Prince of Wales. The Duchy accounts are already under rigorous scrutiny. They are audited by an independent auditor and also subject to scrutiny by the Treasury."

She added that the Prince did not pay corporation tax because he already paid income tax and, in relation to capital gains tax, he is not entitled to any capital gains .

A spokesman at the Treasury added: "We will respond to the subsequent technical enquiries in due course."

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