Three judges on a freedom of information tribunal decided that it is in the public interest "for there to be transparency as to how and when Prince Charles seeks to influence government."
The ruling comes in response to requests by The Guardian newspaper, which has been asking for seven years for the letters to be released.
Several government departments had refused to divulge them, arguing it might breach unwritten constitutional rules on the relationship between the monarchy and the government, and that it would discourage the prince from speaking frankly.
In Britain's constitutional monarchy, the monarch has no political power, but meets regularly with prime ministers and other senior politicians to talk about events of the day. The heir to the throne is entitled to be kept informed of government activities as part of his or her preparations for the throne.
But critics accuse Charles of overstepping boundaries and meddling in political decisions. The 63-year-old prince has strong views on issues including ecology, alternative medicine - both of which he supports- and modern architecture, of which he takes a dim view.
In 2009, leading architects Norman Foster, Zaha Hadid and Frank Gehry accused Charles of using his royal position to attack plans to develop a former army barracks in London into luxury homes.
The tribunal ruling applies only to the letters originally requested by The Guardian, which are from 2004 and 2005.
For centuries, the workings of the British monarchy were shrouded in secrecy, but under freedom of information laws that took effect in 2005, information about the royal family can be released if it is shown to be in the public interest.
That window was partially closed last year when a law took effect making Queen Elizabeth II, Prince Charles and Prince William exempt from freedom of information laws.
The judges noted Tuesday that "in future cases ... there will be severe limitations on the ability to obtain from public authorities information relating to communications with the heir to the throne."