that Prince Charles has received confidential Cabinet papers for decades, making him Britain's "best informed lobbyist".
Confirmation came from the Cabinet's "precedent book", which was locked away "cupboard within a locked office in a secured corridor inside the Cabinet Office".
The Cabinet Office fought for three years against the release of the documents, which came after a request from Republic, the campaign for an elected head of state.
The document reads: "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 (or to be treated as in charge of departments)".
Prince Charles has long been accused of interference in political matters and the Guardian's release of the 'black spider' memos revealed the extent of his lobbying in personal letters to ministers.
It's now revealed that Prince Charles lobbied with full knowledge of Cabinet agendas and policy.
"The disclosure of Cabinet papers to Prince Charles is quite extraordinary," said Graham Smith, Republic's chief executive. "Not only because they would contain highly classified information, but because it gives him considerable advantage in pressing his own agenda when lobbying ministers. He is essentially a minister not attending Cabinet. He gets the paperwork and has private meetings with ministers about policy."
Member of Commons, Paul Flynn, said: "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."
The ''black spider'' memos showed Prince Charles lobbying on Helen Clark's behalf during a funding dispute over a restoration project in the Antarctic, asking for "a bit of imaginative flexibility in the interpretation of these rules".
"I promised Helen Clark I would raise this issue with you - so I have!"
Some of his "pet topics" such as heritage appear benign, but the release shows Charles has access to more top-level information than even the ministers who hold the portfolios in these areas.
Another chapter in the precedent book, "Relations with Buckingham Palace," remains withheld from release.