Prince George has cute name for great-granny the Queen

Her Britannic Majesty has many grand names — Elizabeth Alexandra Mary, Queen of the United Kingdom, Canada, Australia and New Zealand, Head of the Commonwealth among them — but it's a good bet this one is her favorite at the moment: "Gan-Gan."

Prince George, 2½, her great-grandson and future king, third-in-line to her throne, calls his great granny "Gan-Gan," according to his mother, Duchess Kate of Cambridge.

Queen Elizabeth II, who's been accustomed over 64 years on the throne to being addressed as Your Majesty or Ma'am at all times, even by her children, apparently delights in this new moniker.

Duchess Kate reveals this tidbit in a new two-hour TV documentary celebrating the queen's 90th birthday, to be broadcast on Britain's ITV channel over Easter. Teaser clips from the film have come out in the last few days to promote the broadcast, leaving millions of Brits cooing over two of their favorite royals.

Check this ITV story for clips from the documentary, Our Queen at Ninety:

In her first solo interview since marrying Prince William in 2011, the former Kate Middleton says the queen is a doting great-grandmother to Prince George and his baby sister, Princess Charlotte, 10 months, and "she always leaves a little gift or something in their room when we go and stay."

She also says she's grateful to the queen for helping her adjust to life as a member of the royal family.

"She's been very generous in not being forceful at all and in any of her views, but I feel she's been there, a gentle guidance really for me," she says in the film.

Our Queen at Ninety features unprecedented interviews with more of the queen's royal relations than ever before, including her heir, Prince Charles the Prince of Wales, his wife Camilla Duchess of Cornwall, and her grandchildren Prince William, Prince Harry and Princess Eugenie.

"Above all, we will see a monarch who, dare one say it, is not quite as thrilled about a birthday with a round number as everyone else," wrote Robert Hardman, one of the queen's biographers who worked on the documentary, in the Daily Mail. "For if there is one overarching impression after all our filming, it is that the Queen is focusing firmly on business as usual."

Although the queen's actual birthday is April 21, it's officially celebrated in June when the weather is better, and this year will include even more celebrations in May at Windsor Castle and in London.