The verdict – the culmination of a 10-year legal fight by the Guardian – is a significant blow for the government, which has been battling to protect the Prince of Wales from scrutiny over his "particularly frank" interventions on public policy.

In 2012 Dominic Grieve, the then attorney general, said the correspondence contained the prince's "most deeply held personal views and beliefs" and disclosure might undermine his "position of political neutrality" which he might not easily be able to recover when king.

The 27 letters were sent between Charles and ministers in seven government departments in 2004 and 2005. Five of the seven judges in the court ruled in favour of the Guardian's case to see the letters. The verdict was delivered on Thursday by Lord Neuberger, the president of the court.

The judges concluded that Grieve did not have the legal power to veto a freedom of information tribunal, which had decided the memos should be published.

In a statement read out in court one of the supreme court in Westminster, Neuberger said: "We dismiss the attorney general's appeal and the decision of the upper tribunal that the advocacy correspondence should be disclosed under the Freedom of Information Act … stands."

The prime minister and a spokeswoman for Charles said they were disappointed that the ruling had challenged the principle that senior members of the royal family were able to express their views to government confidentially.