LONDON (Reuters) - Britain's Supreme Court ruled on Thursday that 27 "frank" letters written by Prince Charles to ministers in 2004-2005 can be disclosed to the media, a step that could cast doubt over the political neutrality of the future king.
The Guardian newspaper has sought for a decade to obtain the letters sent to ministers under then-Prime Minister Tony Blair, but successive governments have acted to prevent publication for fear of undermining the position of the heir to the throne.
Prime Minister David Cameron called the Supreme Court judgment "disappointing" and said the government would now consider how to release the letters.
"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," Cameron said.
A spokeswoman for Charles at Clarence House, his official London residence, said: "Clarence House is disappointed the principle of privacy has not been upheld."
Under Britain's unwritten constitution, the royal family is supposed to remain politically neutral.
Queen Elizabeth has kept her opinions to herself during her 63-year reign, but Charles has expressed views about subjects close to his heart such as nature conservation and architecture.
His letters, nicknamed "black spider memos" because of his scrawled handwriting, are potentially controversial if they create the perception he disagreed with ministers and tried to influence policies.
Thursday's ruling is an unwelcome public relations setback for Charles, whose aides have worked hard to improve his image after his 1996 divorce from his popular wife Princess Diana and the fallout from her death in a Paris car crash in 1997.
Charles biographer Catherine Mayer, who had unusually free access to him while researching, told Reuters she thought the letters would cause a media furor when they were released but would not harm him in the long term.
"Although I think there will be headlines and there will be embarrassment for him in here and I imagine he's going to be in a deep funk about the whole thing, I don't think this will in the end derail his prospects of ascending the throne," she said.
Then-Attorney General Dominic Grieve, who called the letters "particularly frank", used his ministerial veto in 2012 to block their disclosure, but the veto was declared unlawful by the Court of Appeal last year.
In a final effort to stop disclosure, Grieve went to the Supreme Court, but it dismissed his appeal.
Grieve had argued that disclosure "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".
Alan Rusbridger, editor-in-chief of the Guardian, said he was "delighted".
"The government wasted hundreds of thousands of pounds trying to cover up these letters, admitting their publication would 'seriously damage' perceptions of the prince's political neutrality. Now they must publish them so that the public can make their own judgment," Rusbridger said in a statement.
Only the 27 letters will be released because freedom of information laws have been tightened to prevent publication of correspondence involving the monarch or heir to the throne for 20 years or five years after their death, whichever comes first.
Cameron hinted the law might be changed again as a result of Thursday's ruling.
"Our freedom of information laws specifically include the option of a governmental veto, which we exercised in this case for a reason. If the legislation does not make parliament's intentions for the veto clear enough, then we will need to make it clearer," he said.
Editing by Stephen Addison and Ralph Boulton