Prince Charles routinely receives copies of secret Cabinet papers, it has been revealed.
It came to light following a three-year freedom of information battle between campaign group Republic and the Cabinet Office.
As heir to the throne, Charles is included in a list of government ministers who are on the Cabinet's "standard circulation list".
However, the Cabinet Office' "Precedent Book" states the need for secrecy is so great that Cabinet ministers are handed copies in person.
Republic has now written to the Prime Minister demanding Charles be removed from the circulation list, which includes details of minister's discussions on upcoming legislation normally kept secret for 20 years.
Republic's chief executive Graham Smith said: "The disclosure of Cabinet papers to Prince Charles is quite extraordinary and completely unacceptable.
"This is 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 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.
The Queen is also included on the list but Republic said they were not raising similar concerns over her access to documents because of her position as head of state.