The Prince of Wales has been routinely receiving copies of confidential Cabinet papers, an official document released after a lengthy freedom of information battle showed.
The Cabinet Office's "Precedent Book", drawn up in 1992, says the documents of the Cabinet and ministerial committees are provided to a "standard circulation" list limited to the Queen, the Prince of Wales and government ministers.
Heirs to the throne are believed to have been included in the group since the 1930s.
The book warns that the need for secrecy is so great that "special care in circulation and handling" is required, and Cabinet ministers are handed their copies in person.
Four chapters from the book were released to campaign group Republic after the Cabinet Office failed in a three-year effort to avoid making it public.
Republic has now written to Prime Minister David Cameron demanding that Charles be removed from the circulation list for the papers, which would include details of ministers' discussions on upcoming legislation normally kept secret for at least 20 years.
Republic's chief executive Graham Smith said: "The disclosure of Cabinet papers to Prince Charles is quite extraordinary and completely unacceptable, not only because they would contain highly classified information but because it gives him considerable advantage in pressing his own agenda when lobbying ministers."
The group's letter to Mr Cameron warned that the papers could also include market-sensitive information which could be used to further an individual's financial interests.
The released chapters say: "The documents of the Cabinet and ministerial committees are issued primarily to the sovereign, the Prince of Wales, and ministers. Cabinet documents are issued to ministers personally ... The need for secrecy calls for special care in circulation and handling."
The document adds: "The standard circulation for cabinet memoranda includes the Queen, the Prince of Wales, all members of the cabinet, any other ministers in charge of departments, the attorney general and the chief whip. A few other senior ministers may receive copies at the Prime Minister's discretion ... Ministers of state and junior ministers do not normally receive memoranda."
The revelation follows the release earlier this year, following another FOI battle, of the so-called "black spider letters" sent by the Prince of Wales to ministers over a number of years on subjects ranging from homeopathy to rainforests and defence spending.
In its letter to Mr Cameron, Republic said: "The fact that ... Charles has privileged access to Cabinet papers is a further cause for concern as it means he is able to lobby ministers in secret at every stage of policy development process.
"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 letter warned: "Cabinet papers will include market-sensitive information that would enable a person in possession of the information to use it to further their own financial interests."
Mr Smith said the group was not raising similar concerns over the Queen's access to the documents because of her position as head of state.
"The Queen has a constitutional function, but the Prince of Wales has none, and he is known to be lobbying on a number of issues," said Mr Smith.
He said the Prince's inclusion on the circulation list effectively made Charles "a minister not attending Cabinet".
Mr Smith said: "He gets the paperwork and has private meetings with ministers about policy.
"Charles has no legitimate need to see Cabinet papers at all. His political and private interests and the high degree of secrecy surrounding his lobbying mean there is a real danger this information can be abused without any possibility of accountability."
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."
The disclosure that Charles has been receiving the documents of the cabinet and ministerial committees follows the publication earlier this year of some of his secret letters to government ministers.
The documents disclosed the prince corresponded on a range of topics from tackling the then prime minister Tony Blair over a lack of resources for the Armed Forces fighting in Iraq, to raising the issue of badger culling and the use of herbal medicine.
Twenty-seven letters - 10 from Charles to ministers, 14 by ministers and three letters between private secretaries - written between September 2004 and March 2005 were published.
They were released in May following a 10-year campaign by Guardian newspaper journalist Rob Evans to see the documents following a freedom of information request.
Charles faced criticism that he was lobbying ministers about issues but Clarence House defended the Prince's decision to write the letters.
A spokesman said at the time: "The publication of private letters can only inhibit his ability to express the concerns and suggestions which have been put to him in the course of his travels and meetings.''
A further batch of letters were released in June and showed the prince wrote to ministers about the benefits of complementary medicine, the need for affordable rural homes and the threat to heritage buildings.