Members of the public and journalists will be banned from seeing the contents of secret documents on the Cabinet and Royal Family under measures quietly announced by Gordon Brown last week.
The Prime Minister's reforms on improving parliament contained a little-noticed plan to block Freedom of Information requests on Cabinet papers, even if there is a public interest case.
Graham Smith, of the anti-monarchy campaign Republic, said: "We have been successfully using freedom of information to raise questions about Royal use of public funds and resources and to question [Prince] Andrew's role as trade ambassador. It appears they would prefer to remain in secrecy, and that begs the question why?"