The Attorney General will challenge a Court of Appeal ruling that he 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 Dominic Grieve 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 Mr 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".
Prince 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 under the Freedom of Information Act.
Guardian editor Alan Rusbridger told ITV News:
Clarence House, Prince Charles' office, declined to comment on the issue.