Prince Charles has been receiving confidential Cabinet papers as a matter of course for decades in what campaigners described as a direct violation of the government's own rules. Campaign group Republic have revealed the heir to the throne has been handed documents which detail proposals for upcoming laws, which are usually closed to the public for 30 years.

Following a three-year Freedom of Information battle with the Cabinet Office, Republic was able to make four chapters of the government's "Precedent Book" made public, which reveals Prince Charles, the Queen and other department ministers have been receiving secret documents since at least 1992, when the book was written.

The book itself states that the need for the Cabinet documents to be kept secret is so vital that cabinet ministers are handed their copies in person and ministers of state and junior ministers do not normally receive copies of any such written statements.

Republic have now written to Prime Minister David Cameron urging the government to remove Prince Charles from the list of people who receive the documents.

Graham Smith, Republic's CEO, 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. Charles is essentially a minister not attending cabinet. 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 announcement arrived after Prince Charles' secret letters to minister's and government officials were released following a decade long battle from the Guardian. The so-called 'black spider memos', named because of the Prince of Wales's distinctive handwriting, showed the monarch writing to then senior Labour ministers discussing matters such as the Iraq War, alternative medicine and badger culling.