David Cameron has revealed what he said to Boris Johnson when all six living prime ministers gathered on Saturday.
The former prime ministers were pictured talking to each ahead of a meeting of the Accession Council at St James’s Palace.
They had gathered to witness a centuries old ceremony in which King Charles III was formally proclaimed the new monarch.
Cameron, who was Tory prime minister from 2010 to 2016, told the BBC’s Sunday with Laura Kuenssberg show: “Well, the number of ex-prime ministers is growing.
“I said to Boris it’s the club that no one wants to join and you never get to leave.
“But there’s always a good sort of camaraderie amongst us. It’s interesting because often they were your principal political opponent.
“I was up against Tony Blair and then Gordon Brown and now when we meet at these things, it’s much more talk about how the children or grandchildren are and sort of discussions rather than anything more profound, so that was mostly what the banter was about I think when we were chatting and welcoming in Boris our newest member.”
Asked if he thought Johnson was happy to join the club, Cameron replied: “He was looking happy. I don’t know if he’s happy to join the club.”
Cameron also described how he apologised to the Queen after revealing details of one of their private conversations.
Microphones picked up the then prime minister recounting how the Queen “purred” down the telephone when he informed her of the result of the 2014 Scottish independence referendum.
Cameron added: “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.”
Asked if the Queen had told him off, he replied: “Obviously everything said at those meetings is entirely private.”
During the interview, Cameron described King Charles III as “intelligent and charming” and said he was a “superb diplomat”.
The former MP for Witney also revealed that his wife, Samantha, would often hope to be sat to the then Prince of Wales at official events.
He added: “Like his mother, he is a superb diplomat. I saw him in action at the Commonwealth heads of government meetings.
“He knows every leader personally, he interacts with them brilliantly. 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’ll see that Charles III will be a very worthy successor in that regard.”
Cameron added: “He’s extremely intelligent and charming. My wife always said when we had to go to one of those banquets or dinners or big meetings she always said I hope I’m sitting next to Prince Charles he’s the best person to sit next to, he is the most charming company.
“He’s immensely charming, he’s highly intelligent. While it is incredibly sad that we’re mourning the loss to our greatest monarch, I think the constitutional monarchy is in great hands.”