Government papers released after a three-year freedom of information battle have revealed that Prince Charles receives copies of confidential Cabinet documents.
The heir to the throne, who has previously been accused of "meddling" in politics, has received all cabinet memoranda for decades as a matter of routine.
Confirmation of the procedure came in the Cabinet Office's "precedent book", which was released after a battle by campaign group Republic.
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In a letter to Prime Minister David Cameron, Republic said: "It is plainly wrong that Charles can lobby on new policy proposals even before the public are aware of the existence of such proposals."
The group's chief executive, Graham Smith, said the Prince had "no legitimate need" to see the papers, and called the practice "quite extraordinary and completely unacceptable".
Labour MP Paul Flynn, a member of the cross-party public administration and constitutional affairs committee, which scrutinises the Cabinet office, is demanding a parliamentary investigation.
"He is not just a figurehead, he has become a participant in national debate and there is no control over his lobbying," Flynn told The Guardian.
"This means that he is not only the most influential lobbyist, but the best informed and he is lobbying for his own interests, which are not always benign or sensible."
Flynn's fellow Labour MP and party deputy leader Tom Watson believes the revelation strengthens the case to protect the Freedom of Information Act. "The Conservatives want to water down the act. Labour would strengthen and extend it," he said.
However, Tory MP Bernard Jenkin, chair of the public administration and constitutional affairs committee, said it was "outrageous" to describe Prince Charles as a lobbyist. "This is really a debate about the Prince of Wales' conduct, not what papers he sees", he said.
He also argued that the "vast majority" of people would agree that the heir to the throne should have sight of such documents, a view echoed by Tory MP Michael Ellis.
He claimed: "It is obvious that the heir to the throne needs to have access to certain relevant cabinet papers and other documents so he can prepare for the role of Sovereign in due course.
"The Prince of Wales has proven himself to be a powerful advocate on the side of the British people and he is one of our most well-informed and expert individuals on a wide range of issues."
Discussing the news, a Cabinet Office spokesman said: "It has been established practice for many years that the sovereign and the heir to the throne receive the minutes of Cabinet meetings. It is important that the head of state and her heir are properly briefed."
