Security protection officers and staff also refer to the Queen as ‘S’ in written communications, which is thought to be a reference to her being the Sovereign.
According to The Sun, the code name was revealed by a royal aide during a visit to cathedral town on reconnaissance ahead of a royal visit.
A source reveals the aide to have said: "She's written down as 'S' before cupping his hand over his mouth and whispering "it's Sharon" when pressed on what "S" stood for.
"It is highly likely. The purpose of any sobriquet is that it should be anonymous and memorable,” royal historian Hugo Vickers told the publication.
"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.
Prince Charles is also said to have affectionately nicknamed daughter-in-law Meghan 'Tungsten', because, just like the super-tough metal, she's "tough and unbending".
Meanwhile, Princess Diana is said to have sometimes called a young Prince William ‘Wombat’.
"It began when I was two. I've been rightfully told because I can't remember back that far,” William explained.
"But when we went to Australia with our parents, and the wombat, you know, that's the local animal."