Details of secret powers held by senior members of the Royal Family granting veto over Government legislation could me made public after a decision by the Information Commissioner.
The Cabinet Office has been told to hand over papers which guide ministers on when and how to consult the Queen and Prince Charles over new laws.
It follows a Freedom of Information request by campaigners amid mounting concern over the Prince of Wales's intervention in public life on issues ranging from architecture to nanotechnology.
The Cabinet Office said it was considering whether to appeal the decision.
The Prince has been asked to consent to at least 12 draft bills during the past two Parliaments. Among the issues are those relating to his role as head of the Duchy of Cornwall, which provides his £17m annual private income.
Under the arcane system, the Duchy, one of the UK's biggest landowners, is concerned with legislation affecting issues such as marine and coastal access, planning and energy schemes. The Prince's views have also been sought on matters referring to coroners courts.
The Duchy and the Cabinet Office have fought to keep documents advising ministers on protocol secret, insisting the role of senior royals is procedural rather than substantive.
A Buckingham Palace spokesman said: "The case in question involves material held by the Cabinet Office and is a matter for the Government."Reuse content