Ministers have been forced to seek permission from Prince Charles to pass at least a dozen government bills, according to a Guardian investigation into a secretive constitutional loophole that gives him the right to veto legislation that might affect his private interests.

Since 2005, ministers from six departments have sought the Prince of Wales' consent to draft bills on everything from road safety to gambling and the London Olympics, in an arrangement described by constitutional lawyers as a royal "nuclear deterrent" over public policy. Unlike royal assent to bills, which is exercised by the Queen as a matter of constitutional law, the prince's power applies when a new bill might affect his own interests, in particular the Duchy of Cornwall, a private £700m property empire that last year provided him with an £18m income.

Neither the government nor Clarence House will reveal what, if any, alterations to legislation Charles has requested, or exactly why he was asked to grant consent to such a wide range of laws.

Correspondence seen by the Guardian reveals that one minister wrote to the prince's office requesting his consent to a new bill about planning reform because it was "capable of applying to ... [the] Prince of Wales' private interests".

In the last two parliamentary sessions Charles has been asked to consent to draft bills on wreck removals and co-operative societies, a freedom of information request to the House of Commons has revealed. Between 2007-09 he was consulted on bills relating to coroners, economic development and construction, marine and coastal access, housing and regeneration, energy and planning.

MPs and peers called for the immediate publication of details about the application of the prince's powers which have fuelled concern over his alleged meddling in British politics. "If princes and paupers are to live as equals in a modern Britain, anyone who enjoys exceptional influence or veto should exercise it with complete transparency," said Andrew George, Liberal Democrat MP for St Ives in Cornwall. "The duchy asserts that it is merely a private estate. Most people will be astonished to learn that it appears to have effective powers of veto over the government."

"We should know why he is being asked and the government should publish the answers," said Lord Berkeley, who was last month told to seek Charles' consent on a marine navigation bill. "If he is given these powers purely because he owns land in Cornwall it is pretty stupid. What about the other landowners who must also be affected by changes to legislation?"

Revelations about Charles' power of consent come amid continued concern that the heir to the throne may be overstepping his constitutional role by lobbying ministers directly and through his charities on pet concerns such as traditional architecture and the environment.

A spokesman for the Prince of Wales would not comment on whether the prince has ever withheld consent or demanded changes to legislation under the consent system. "Communications between the prince or his household and the government are confidential under a long-standing convention that protects the heir to the throne's right to be instructed in the business of government in preparation for his future role as monarch," he said. Daniel Greenberg, a former parliamentary counsel and now parliamentary lawyer at Berwin Leighton Paisner, said: "It is something of a nuclear-button option that everybody knows he is not likely to push. But like the nuclear deterrent, the fact that it is there, influences negotiations."

Graham Smith, director of Republic, the campaign for an elected head of state, said it was "an affront to democratic values" that citizens had no right to know whether Charles was insisting on changes to bills. "We know Charles has been lobbying ministers, but this is evidence he has the power to instruct them to alter their plans and that gives him leverage," he said.