The Attorney General is to challenge a Court of Appeal ruling that he has unlawfully prevented the public seeing letters the Prince of Wales wrote to government ministers.
Lord Dyson, the Master of the Rolls, and two other appeal judges opened up the possibility that the royal correspondence could soon be revealed to the public gaze.
They unanimously ruled that the Government's principal legal adviser has "no good reason" for using his ministerial veto and overriding the decision of an independent tribunal, chaired by a High Court judge, in favour of disclosure.
The case is believed to mark the first time that anyone has challenged the Attorney General's powers to block access to information.
A spokesman for Dominic Grieve said he will now take his case to the Supreme Court, highest court in the land, "in order to protect the important principles which are at stake in this case".
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 hand-written letters he pens to government ministers expressing his views .
Guardian journalist Rob Evans applied to see a number of written communications between Charles and various government ministers between September 2004 and April 2005.
Mr Evans sought disclosure under the Freedom of Information Act 2000 (FOIA), and under the Environmental Information Regulations 2004.
The Upper Tribunal (UT) declared in September 2012 that he was entitled to see "advocacy correspondence", described as letters Charles had written seeking to advance the work of charities or to promote views.
The seven departments Charles wrote to are Business, Innovation and Skills; Health; Children, Schools and Families; Environment, Food and Rural Affairs; Culture, Media and Sport; the Northern Ireland Office and the Cabinet Office.
A month after the UT ruling, the Attorney General used his ministerial veto by issuing a certificate under Section 53 of the FOIA.
He said the public could interpret the letters sent to ministers in the last Labour government as showing Charles to be "disagreeing with government policy".
Mr Grieve 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".
In his opinion, Government departments were legally entitled to refuse disclosure because the correspondence was undertaken as part of the Prince's "preparation for becoming king".
Mr Evans accused Mr Grieve of failing to show "reasonable grounds" for blocking disclosure.
He lost his case in the High Court, but appeal judges Lord Dyson, Lord Justice Richards and Lord Justice Pitchford all agreed that Mr Grieve had gone wrong in law and the Section 53 certificate should be quashed.
Lord Dyson ruled: "I do not consider that it is reasonable for an accountable person (the Attorney General) to issue a Section 53(2) certificate merely because he disagrees with the decision of the tribunal. Something more is required.
"Examples of what would suffice are that there has been a material change of circumstances since the tribunal decision or that the decision of the tribunal was demonstrably flawed in fact or in law."
The Attorney General's actions were also incompatible with European law, said the judge.
Mr Grieve's spokesman said later: "We are very disappointed by the decision of the court. We will be pursuing an appeal to the Supreme Court in order to protect the important principles which are at stake in this case."
A Guardian News & Media spokeswoman said: "The public has a right to know if the heir to the throne is advocating policy or promoting causes to government ministers.
"We welcome today's appeal court judgement finding that it was wrong to block the release of the letters.
"We hope the Attorney General will recognise he has reached the end of the legal road and that government departments will now publish the correspondence so that the public can judge for themselves."
Clarence House, Charles's office, declined to comment.