The Queen was not in favour of Prince Charles becoming Australia's governor-general, at least until he had "a lady by his side".
There was talk in 1976 about the Prince of Wales becoming governor-general, following the controversial 1975 dismissal of the Whitlam government.
But in a letter to Governor-General Sir John Kerr, the Queen's private secretary Sir Martin Charteris revealed she would not entertain the idea until the heir-to-the-throne was married.
"I think the point we must all bear in mind is that I do not believe The Queen would look with favour on Prince Charles becoming Governor-General of Australia until such time as he has a settled married life," Sir Martin wrote in August 1976.
"No one will know better than you how important it is for a governor-general to have a lady by his side for the performance of his duties.
"The prospect, therefore, of The Prince of Wales becoming Governor-General of Australia must remain in the unforeseeable future."
In another letter, Sir Martin noted that apart from the considerations of a settled married life, it would be very difficult for Prince Charles to become governor-general until wider constitutional questions were solved.
Letters between Sir Martin and Sir John, released on Tuesday, reveal Prince Charles was being briefed on Australia's political crisis before the governor-general sacked Gough Whitlam in November 1975.
Sir John told the Prince of Wales that he believed Mr Whitlam was moving to sack him as governor-general, according to an October 1975 letter from Sir Martin to Sir John.
The Queen wanted Sir John to know she had received a very full account from Prince Charles of their talks, Sir Martin wrote.
"Prince Charles told me a good deal of his conversation with you and in particular that you had spoken of the possibility of the Prime Minister advising The Queen to terminate your commission with the object, presumably, of replacing you with somebody more amenable to his wishes," he wrote.
"If such an approach was made you may be sure that The Queen would take most unkindly to it."
Sir Martin, however, noted the Queen would have no option but to follow the advice of her prime minister.
A September 1975 letter also noted Prince Charles had briefed the Queen on the problems Sir John was facing, after returning from a trip to Papua New Guinea to mark its independence.
The so-called palace letters also revealed Prince Charles was keen to buy a property in NSW in 1974, but it was decided that would not be a good look for the monarchy.
"It is felt that the public in this country would misunderstand a decision by The Prince of Wales to buy a property at a time of great economic difficulty for the United Kingdom and when housing is one of the worst problems which faces ordinary people," Sir Martin wrote in October 1974.
"In modern times it is never 'a good moment' for The Royal Family to spend money, but I think it fair to say that the present could hardly be a worse moment."