Britain's top court on Thursday overruled the government to allow the publication of a series of letters from Prince Charles to ministries, in a potential setback for the heir to the throne.

"The Supreme Court dismisses the attorney general's appeal," read the ruling, after the government went to court to prevent the publication of letters believed to show Charles interfering in politics.

A spokesman for Clarence House, the prince's residence, said it was "disappointed the principle of privacy has not been upheld".

The long-running case was led by a journalist for The Guardian and the newspaper's editor Alan Rusbridger hailed a "brilliant 10-year campaign".

"Guardian wins," he wrote on Twitter.

The 27 handwritten letters have been dubbed the "Black Spider Memos" due to Charles's spiky handwriting.

No date has been fixed for their publication.

"This is a disappointing judgement and we will now consider how to release these letters," Prime Minister David Cameron said in a statement.

"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," he said.

The government had vetoed the release of the letters under Freedom of Information laws, but this has now been struck down by the courts.

Cameron said the government would consider amending the law.

"If the legislation does not make parliament's intentions for the veto clear enough, then we will need to make it clearer," he said.

A new biography of Charles last year reignited debate about whether he is fit to become king, given his outspoken views and energetic activism on a variety of issues including the environment and architecture.

Catherine Mayer's "Charles: The Heart of a King" also portrays a royal household riven with infighting, and an heir to the throne uncomfortable with the distant impartiality that has been the hallmark of his mother Queen Elizabeth II's reign.

The 66-year-old Prince of Wales, Queen Elizabeth's eldest son, has spent a lifetime in preparation for the throne.

But he has also carved out a distinct and highly visible role for himself in public life by wading into topics that interest him.