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.
They unanimously ruled that the Government's principal legal adviser had "no good reason" for using his ministerial veto.
The case is believed to mark the first time anyone has challenged the Attorney General's powers to block access to information.
A spokesman for Dominic Grieve said he would now take his case to the Supreme Court "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.
The Attorney General had earlier 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".
Clarence House, Prince Charles's office, declined to comment.