When asked what the "S" stands for, he reportedly cupped his hand over his mouth and whispered: "Sharon".
Royal historian Hugo Vickers has also confirmed that the rumoured nickname is "highly likely".
"The purpose of any sobriquet is that it should be anonymous and memorable," he told The Sun.
"If it can also be a bit mischievous and therefore all the more memorable, then that makes sense.
"Although S might simply stand for Sovereign of course."
Last year another one of the Queen's nicknames was revealed.
According to columnist Richard Kay, Prince William, as a young child, began bawling "Gary, Gary" one time after he fell over in Buckingham Palace.
When the Queen picked up her grandson, someone asked her who Gary was. "I'm Gary. He hasn't learned to say granny yet."
The rest of the royal family also reportedly has nicknames for safety precausions.
To members of the public they are known as Prince Harry and Meghan Markle, but to their royal aides and bodyguards they are just plain "David Stevens" and "Davina Scott".
It has been revealed that the famous couple are regularly given alternative monikers – secret code names that use the initials "DS" as a nod to their official title, the Duke and Duchess of Sussex, to prevent their contact details from falling into the wrong hands.
And as for Prince William and Kate Middleton, the couple are saved as the more modest "Danny Collins" and "Daphne Clark" in their aides' contact lists, Reports The Daily Mail.
The initials "DC" in the secret code names allude, of course, to the royal couple's title – the Duke and Duchess of Cambridge, reports the Daily Star.
