The Supreme Court has ruled that letters sent by Prince Charles to government ministers can legally be published, after a Guardian-led campaign.

The UK’s highest court ruled five to two that the Attorney General’s office acted unlawfully when it prevented the letters’ publication in 2012.

Clarence House said that it was “disappointed the principle of privacy had not been upheld”.

David Cameron was also disappointed at the ruling, but said that the government would now decide how best to release the letters.

The prime minister 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.”

The verdict is the culmination of a 10-year campaign by the Guardian, which first requested copies of the so-called “black spider memos”, 27 letters sent between the prince and government ministers between 2004-5, in a Freedom of Information request.

In blocking the Guardian’s request to publish the letters, Attorney General Dominic Grieve said that the letters held the prince’s “most deeply held personal views and beliefs” and any perception the prince had disagreed with the then Labour government “would be seriously damaging to his role as future monarch because if he forfeits his position of political neutrality as heir to the throne, he cannot easily recover it when he is King”.

The royal’s own website makes clear that as a constitutional monarchy, the reigning monarch should remain politically neutral. As heir to the throne, many queried the influence he should have over the running of the country and why his correspondence should remain privileged.