LONDON—The Guardian has won a court battle in its efforts to disclose letters by Prince Charles — a decision the newspaper argues could shed light on whether he has used his position to influence politics.

The Court of Appeal ruling Wednesday came after Attorney General Dominic Grieve refused to let the public see Charles’ correspondence with seven U.K. government departments. Grieve had argued the particularly frank letters reflect the personal views of Charles, who is first in line to the throne. The fear is Britons may not find Charles to be politically neutral — as a king must be — and the monarchy would be undermined.

The Guardian has campaigned for letters’ release, arguing the government failed to show reasonable grounds for them to be blocked.

Grieve’s office will appeal.