The "opinionated" letters written by Prince Charles to the British government will remain secret, Britain's Attorney General, Dominic Grieve, ruled today.
The decision overrules three judges who decided last month that the public had a right to see what sort of advice the prince was offering to government ministers.
Grieve described the letters as "particularly frank," and said Britons reading them might not think the prince is politically neutral, as a monarch must be, ABC News reported.
"Much of the correspondence does indeed reflect the Prince of Wales' most deeply held personal views and beliefs," Grieve said in a statement.
The letters "contain remarks about public affairs which would in my view, if revealed, have had a material effect upon the willingness of the government to engage in correspondence with the Prince of Wales, and would potentially have undermined his position of political neutrality."
The Guardian reported the battle for access to the letters began in 2005 when the Guardian submitted freedom of information requests to see the prince's letters to ministers over a seven-month period.
It followed widespread criticisms of the prince for "meddling" in government affairs and seeking to persuade ministers to change policy.
He is believed to write to ministers arguing his personal point of view and expressing his opinion.
Critics have long held the view the prince should stay out of government policy because he is not democratically elected.