Prince Charles sent a hand-written letter of support to Australia's governor general in 1976, backing his controversial sacking of Prime Minister Gough Whitlam, Australian media reported.
The letter, published by The Australian newspaper, is dated four months after Queen Elizabeth's representative in Australia, John Kerr, took the unprecedented step to dismiss Whitlam without first warning the palace or the prime minister.
"Please don't lose heart," the heir to the British throne wrote in the hand-written letter to Kerr on 27 March.
"What you did last year was right and the courageous thing to do - and most Australians seemed to endorse your decision when it came to the point."
The letter was revealed in an extract of a book The Truth of the Palace Letters: Deceit, Ambush and Dismissal in 1975 by Paul Kelly and Troy Bramston, due to be published next month.
Whitlam's firing remains one of the country's most polarising political events because it represented an unmatched level of intervention by the Commonwealth.
Historians say the country was never told the full story behind Whitlam's removal during a political deadlock over the Budget and in 2016, one historian sued Australia's National Archives for access to letters between Kerr and the Queen.
In July, the 211 so-called "palace letters" were published, pulling the veil from one of the great mysteries of Australian politics, and re-igniting a conversation about whether the country should cut ties with Britain and become a republic.