An extract from a souvenir brochure from Charles’ 1969 investiture as the Prince of Wales reportedly confirmed the royal revelation.
The brochure stated: “At eight months [Charles] was taken by his parents to their new home, Clarence House.
“His mother spent as much time with her son as her duties allowed and part of the day was always devoted to him.
“Often, he was taken to visit Queen Mary – whom he called Gan-Gan – at Marlborough House and the Queen – later known as the Queen Mother – as Buckingham Palace.”
Speaking during her first solo interview, Kate Middleton also confirmed that George had also adopted the sweet name for his grandmother.
"George is only two-and-a-half and he calls [the Queen] Gan-Gan,” Kate said at the time.
“She always leaves a little gift or something in their room when we go and stay and that just shows her love for her family,” she added.
Kate and William recently made a cheeky confession about how they keep their children focused while homeschooling them amid the coronavirus pandemic.
Speaking during an interview with BBC News, the Duchess admitted that she and the Duke haven’t been entirely honest with their brood, when it comes to the school calendar.
"We don't actually tell the children we've actually kept going through the holidays. I feel very mean," Kate admitted to the British publication.