The government has been accused of orchestrating a "royal cover-up" after it vetoed the publication of letters sent to ministers by Prince Charles.
Last month an appeals court overruled a decision by the Information Commissioner to block a Freedom of Information request submitted by The Guardian.
Granting the appeal, the court said it was in the public interest to discover if and how the unelected prince was attempting to influence government policy.
But on Tuesday Attorney General Dominic Grieve decided to veto the publication of the letters, arguing it could damage Prince Charles' "ability to perform his duties when he becomes King" if they were made public.
"My view is that the public interest favours non- disclosure. I have also concluded that this constitutes an exceptional case and that the exercise of the veto is warranted," he said.
"My decision is based on my view that the correspondence was undertaken as part of The Prince of Wales’ preparation for becoming King.
"The Prince of Wales engaged in this correspondence with Ministers with the expectation that it would be confidential. Disclosure of the correspondence could damage The Prince of Wales’ ability to perform his duties when he becomes King.
He added: "It is a matter of the highest importance within our constitutional framework that the Monarch is a politically neutral figure able to engage in confidence with the Government of the day, whatever its political colour.
The prince's letters are said to be known as "black spider memos" within Whitehall because of his style of handwriting.
The campaign group Republic, which wants to replace the Monarchy with an elected head of state, said the decision showed the prince had something to hide and that ministers have colluded and conspired with him to keep his secrets under wraps.
Republic's chief executive Graham Smith said: "It's an open secret that prince Charles lobbies the government. What the public has a right to know is what he is lobbying for and whether he is actually influencing policy."
"The Attorney General's decision is all about protecting Charles and the royal family from scrutiny, putting his demands above the rights of the British people.
"Dominic Grieve has made it clear today that no citizen should ever bother trying to find out what the royals are doing behind closed doors: the government will never let the light in."
"This decision is a serious affront to British democracy and must be challenged," he said."Grieve has said this is about protecting prince Charles's impartiality, but that impartiality doesn't exist."
The FoI request had been made in order to see the letters between Prince Charles and the Department for Business, Innovation and Skills, the Department of Education, the Department for Environment, Food and Rural Affairs, the Department of Health, the Department for Culture, Media and Sport, the Northern Ireland Office and the Cabinet Office.