Igor Judge, the lord chief justice of England and Wales, said Attorney General Dominic Grieve has the legal standing to veto public access to the prince's letters, The Guardian reported. But Judge also called the veto a "constitutional aberration."
Queen Elizabeth and Prince Charles, as heir to the throne, are expected to be politically neutral. The queen meets regularly with the prime minister, but their discussions are never disclosed.
Charles has taken an active interest in some matters, including architecture, preserving the British landscape and the environment.
His letters to ministers are known as "black spider memos" because of his handwriting, the Guardian said. Ministers have said they include his "most deeply held personal views and beliefs" and making their contents public might make his reign more difficult when he becomes king.
"This risk will arise if, through these letters, the Prince of Wales was viewed by others as disagreeing with government policy. Any such perception 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," Grieve said.
The Guardian said it would appeal with a spokeswoman describing publication of the letters as "squarely in the public interest."