The Supreme Court says letters by Prince Charles to the government can be published, after a Guardian campaign.
The UK's highest court was asked to judge whether the Attorney General's office acted unlawfully when it prevented their publication in 2012.
The newspaper sought disclosure of the letters, written to seven government departments between 2004-5.
The prince's office at Clarence House said it was "disappointed the principle of privacy had not been upheld".
A spokeswoman also said the issue was "a matter for the government".
It has been argued that releasing the so-called "black spider memos" - a reference to the prince's handwriting - would undermine his neutral political status.
Politically embarrassing
Prime Minister David Cameron called the ruling "disappointing" and said the government would now consider how best to release the documents.
He added: "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."
Mr Cameron also hinted that the legislation could need tightening in the wake of the ruling.
"Our FOI (Freedom of Information) 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."
Campaign for Freedom of Information director Maurice Frankel welcomed the decision to release the letters but expressed concern that any move now to strengthen the veto could see it extended to more politically embarrassing cases.
"That would be very unwelcome," he said.
"It gives ministers the opportunity to overturn the wheelbarrow every time they don't like a decision."
On the ruling to release the letters, he added: "This is a critical decision which strengthens the FOI Act. It says the courts, not ministers, normally have the last word."
