Prince Philip said he believed Australians had not “got over the fact that Britain was not able to protect them” during the Second World War, in a letter to a friend which is up for auction.
Philip, now 99, is known for gaffes which have sometimes offended nations, and in the letter from 1965, he turned his sights on Australia, after a royal tour there alongside his wife.
The Duke of Edinburgh also used a slur in the letter, sent to his friend Sir Harold Hartley, as he described how the visit down under had gone.
He said: “Australia is a fascinating place. They're absolutely mesmerised by the Americans and yet they take it as a personal insult that Britain should be in such a muddle at the moment.
“Australian nationalism is growing rapidly and naturally it is directed almost entirely at Britain. They still haven't got over the fact that Britain was not able to protect them from the Japs, in spite of the fact that she was fighting for her life in Europe.
“Therefore their gratitude to the US is almost excessive...”
Sir Harold was a chemist who was chairman of the British Overseas Airways Corporation, who died in 1972.
A speech at his memorial service described him as someone who “served Dulwich and Balliol with a loyalty that was fervent and painstaking”.
Author Tim Heald described Sir Harold as a “loyal and zealous adviser” to the duke, who was an “unquestioned influence” on his life, in his book The Duke: Portrait of Prince Philip.
Recalling events in Australia, the prince was rather disparaging of a man thought to be Sir Henry Bland, who was a public servant in the country at the time of the visit.
Philip described him as “a small dark balding man with large eyes and a rubbery face”.
Read more: Who was Prince Philip's other royal family?
He added: “He seemed quite friendly and obviously rather pleased with himself. It soon became apparent that his understanding of the conference concept and method was superficial and I'm fairly certain he was surprised to find that I knew anything about it at all! I buttered him up and he was soon purring!”
The duke went to Australia at the end of February 1965 and one of his engagements there was to open the Australian mint.
He and the Queen have made many visits to Australia over the years, through her role as head of the Commonwealth.
Their last visit there together was in 2011 for the Commonwealth Heads of Government Meeting (CHOGM).
Perhaps one of his most famous ‘gaffes’ was in 2002 when he asked an aboriginal leader if they still “threw spears at each other”.
In 1974, he finished off the Queen’s tour of Australia when she was called back to the UK because of a general election.
The letter is up for auction with International Autograph Auctions Europe, and is expected to fetch €100 - €150 (£89-£134).