The government has been ordered to release secret internal guidelines on when the consent of the Prince of Wales and the Queen is required for legislation.
The Information Commissioner's Office has ruled that the Cabinet Office must release details of two pamphlets spelling out when officials drawing up new laws need to consult Charles in his capacity as Duke of Cornwall.
It follows a request under the Freedom of Information (FoI) Act for the release of any guidance on obtaining the consent of The Crown and the Duchy of Cornwall before bills are passed into law.
The government has to release documents spelling out when Prince Charles' permission is needed for law
The Cabinet Office argued that the pamphlets - dating from November 2010 and August 2008 - fell into the category of "legal advice privilege" and were not discloseable under the legislation.
"The primary motivation ... was to provider drafters with indicators to assist them in determining whether any part of a Bill might require the consent of the Duchy of Cornwall and should therefore be brought to the attention of the House authorities," he wrote.
The decision was welcomed the John Kirkhope, the legal scholar who submitted the original FoI request.
He told The Guardian: "It was clearly in the public interest that citizens understand how laws are made and applied as well as the circumstances in which the Duchy of Cornwall is consulted."
The Cabinet Office now has until September 25 to release the information or to lodge an appeal.
A spokesman said: "The Cabinet Office has received the Information Commissioner's decision. We will be studying it closely and take a decision on whether to appeal in due course."