David Cameron has revealed he practiced holding weekly audiences with King Charles when he was the Prince of Wales to prepare him for taking over.
The Conservative former Prime Minister said the new monarch had served the "longest apprenticeship in history" and he believed he would be "brilliant" in the role.
The Prime Minister of the day holds a private weekly audience with the monarch to discuss matters of Government.
The meetings, held at the Audience Room in Buckingham Palace, are a key part of the PM's diary and provided the 15 leaders who served during the Queen's reign with an opportunity to seek her counsel.
Mr Cameron disclosed that he had held practice sessions with the-then Prince of Wales when he was still in office.
"I had audiences with Prince Charles when Queen Elizabeth II was on the throne because he wanted to start thinking about how to conduct those audiences," Mr Cameron told the BBC's Sunday With Laura Kuenssberg.
"From what I saw he will be brilliant at that job. Brilliant at listening, brilliant at asking questions, giving wise advice and sage counsel.
"This has probably been the longest apprenticeship in history."
Mr Cameron said the new King was a "superb diplomat", like his mother, and predicted he would prove a "very worthy successor" on representing Britain abroad.
"I saw him in action at Commonwealth heads of government meetings and he knows everybody personally, he interacts with them brilliantly," Mr Cameron said.
"The soft power that the British monarch brings to help a prime minister and a government with all those international relations, it was obviously outstanding under Queen Elizabeth II.
"I think you will see Charles III will be a very worthy successor in that regard."
King Charles held an audience with new Prime Minister Liz Truss on Friday at Buckingham Palace.
All six living ex-PMs were present to watch the new monarch proclaimed as head of state on Saturday at the Accession Council.
Mr Cameron joked that they welcomed Boris Johnson to the club of former PMs after he left No10 on Tuesday.
He said: "The number of ex-PMs is growing. I said to Boris, 'It's the club that no one wants to join and you never get to leave.'"
Mr Cameron cracked a smile when asked if his long-time rival was happy to be in the club of former PMs.
The former PM said: "He was looking happy but I don't know if he was happy to join the club."
Mr Cameron also described how he had to apologise to the Queen after revealing she "purred" down the telephone when he informed her of the result of the 2014 Scottish independence referendum.
He said: "It was a very upfront and fulsome apology done very quickly at the beginning of an audience. I think that is all I should say.
"From ever onwards I have been more careful when cameras and microphones are around and I have learned my lesson."
