Labour MP Paul Flynn demanded a probe into "income support for millionaires" yesterday following revelations that Prince Charles's Duchy of Cornwall is exempt from corporation tax.

Mr Flynn called for full-scale scrutiny of how the royals rake in their cash, including the £19.1 million surplus chalked up by the Duchy of Cornwall over the past year.

"It would be indefensible to have a form of income support for millionaires," said the Newport West MP.

"We should encourage transparency and enable the fabulously wealthy royal family to stand on their own feet."

He also demanded publication of a detailed breakdown of the costs for protecting the royals from attack.

Security costs varied from millions of pounds for a state opening of Parliament, to around £10,000 a time for a thinly attended minor royal event "where the crowds are standing nil deep on the pavement," he said.

Mr Flynn seized on a report from the Commons committee of public accounts which revealed that Prince Charles paid just £4.4m in "voluntary" tax and VAT last year.

In 2012-13 the prince received an income of £19.9m from the Duchy of Cornwall, which operates as a commercial enterprise. But he paid no corporation tax.

He raked in a further £1.2m from "public sources."

However, the prince cheerfully claimed £11m costs for the performance of official duties by himself, the Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry.

The MPs' committee urged more transparency over the prince's tax payments and called on the Treasury to publish more information about its scrutiny of the Duchy of Cornwall.