Letters from Prince Charles to government ministers are set to be released, after a nearly decade-long battle to stop the heir to the throne's "most deeply held personal views" becoming public.
The government has fought attempts to get the letters released every step of the way, arguing their release would damage his ability to be king, where he would be expected to be neutral.
Supreme Court justices in London rejected a challenge by the Attorney General, the Government's principal legal adviser, against a decision by Court of Appeal judges that he has unlawfully prevented the public seeing the letters.
Reacting to the Supreme Court's decision on the letters, Clarence House said it was "disappointed the principle of privacy had not been upheld".
The ruling was met with ecstatic, jubilant reaction from journalists, lawyers and freedom of information campaigners.
Supreme Court President Lord Neuberger said there was a "fundamental composite principle" behind the court's reasons for dismissing the appeal.
He announced: "That principle is that a decision of a judicial body should be final and binding and should not be capable of being overturned by a member of the executive."
Clarence House reacted with disappointment to the decision.
A spokeswoman for the Prince said: "This is a matter for the Government. Clarence House is disappointed the principle of privacy has not been upheld."
The move is a major blow for Charles.
The notes, which were sent to seven government departments between September 2004 and March 2005, reflect, according to previous attorney general Dominic Grieve, the Prince's ''most deeply held personal views and beliefs''.
Charles has long been accused of ''bombarding'' ministers with ''black spider'' memos attacking government policy. His letters are so named after his distinctive handwriting and abundant use of underlining and exclamation marks.
Charles is known for his strong opinions on a range of topics from the environment and farming to complementary medicine and architecture.
He has faced accusations in the past of "meddling" in day-to-day politics and criticism over his "black spider memos" - the name given to the handwritten letters he pens to government ministers expressing his views.
Using his ministerial veto to block the letters' release, the Attorney General said the public could interpret the letters sent to ministers in the last Labour government as showing Charles to be ''disagreeing with government policy''.
He said any perception that Charles had disagreed with Tony Blair's government "would be seriously damaging to his role as future monarch because, if he forfeits his position of political neutrality as heir to the throne, he cannot easily recover it when he is king''.
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In 2005, the Guardian journalist Rob Evans asked to see the letters, which were written between Charles and various government ministers between September 2004 and April 2005.
Alan Rusbridger, editor-in-chief of Guardian News & Media, said: "We are delighted the Supreme Court has overwhelmingly backed the brilliant 10-year campaign by Guardian reporter Rob Evans to shine daylight on the letters Prince Charles has been writing to ministers.
"The Government wasted hundreds of thousands of pounds trying to cover up these letters, admitting their publication would 'seriously damage' perceptions of the Prince's political neutrality. Now they must publish them so that the public can make their own judgment.
"This is a good day for transparency in government and shows how essential it is to have a fully independent judiciary and free press."
David Cameron said: "This is a disappointing judgment and we will now consider how to release these letters.
"This is about the principle that senior members of the Royal Family are able to express their views to government confidentially. I think most people would agree this is fair enough.
"Our FOI laws specifically include the option of a governmental veto, which we exercised in this case for a reason. If the legislation does not make Parliament's intentions for the veto clear enough, then we will need to make it clearer."