The Attorney General has stepped in to overturn plans to publish letters written by the Prince of Wales to several Labour ministers in the last government.
Dominic Grieve said the correspondence between Charles and seven departments of state was part of his "preparation for kingship".
ITV News' Royal Correspondent Tim Ewart reports:
Mr Grieve said making the letters public could potentially damage the principle of the heir to the throne being politically neutral, and so undermine his ability to fulfil his duties when king.
He issued a certificate under the Freedom of Information Act vetoing the disclosure of the letters, which were sent across seven months in 2004 and 2005.
Three judges at a freedom of information tribunal had previously ruled in favour of Rob Evans, of the Guardian, who had challenged a decision by the Information Commissioner to uphold a refusal by the Government departments to release the correspondence.
But in a written statement the Attorney General today said:
I consider that such correspondence enables the Prince of Wales better to understand the business of government; strengthens his relations with ministers; and enables him to make points which he would have a right - and indeed a duty - to make as monarch.
If such correspondence is to take place at all, it must be under conditions of confidentiality. Without such confidentiality, both the Prince of Wales and ministers will feel seriously inhibited from exchanging views candidly and frankly, and this would damage the Prince of Wales' preparation for kingship.
A Guardian News and Media spokeswoman said they intended to take the case further and challenge the veto in the High Court.