LONDON (REUTERS) - The possible publication of potentially embarrassing letters written by British heir-to-the-throne Prince Charles moved a step closer on Wednesday when a gagging order on them was declared unlawful by a court.

The 27 letters were written by Charles to members of the previous Labour government and have been described by Attorney General Dominic Grieve as "particularly frank". The 65-year-old prince has long held strong views in areas like the environment and urban planning and has been criticised for apparently using his unelected position to persuade ministers to change official policies through private letters, nicknamed "black-spider memos" because of his scrawled handwriting.

Mr Grieve, the government's chief legal adviser, has blocked an attempt by the left-leaning Guardian newspaper to have the letters made public under freedom of information laws.

He has said that any perception that Prince Charles had disagreed with the previous government of Tony Blair "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".

On Wednesday, the Appeal Court ruled that Mr Grieve's ban on publication was unlawful but gave him permission to appeal to the Supreme Court.

Grieve's office confirmed it would indeed be appealing.

"We are very disappointed by the decision of the Court," it said in a statement. "We will be pursuing an appeal to the Supreme Court in order to protect the important principles which are at stake in this case." Prime Minister David Cameron is backing Mr Grieve's efforts to ban publication.

"It is important to preserve confidentiality of communications between the monarch and the heir to the throne," his spokesman said on Wednesday.

"(Cameron) very much supports the case the government has been making in the courts and we shall continue to make that case."

The Guardian welcomed Wednesday's ruling, adding in a statement: "The public has a right to know if the heir to the throne is advocating policy or promoting causes to government ministers." A spokesman for Prince Charles declined to comment.