The Queen takes her own box of chocolates when she goes to stay with friends as a guest, as well as a box for the host.
Lady Pamela Hicks, a lifelong friend of the Queen and former lady-in-waiting, has revealed secrets of her life in a one-off documentary.
Lady Pamela, who is a cousin to Prince Philip, revealed that the Queen always takes a box of chocolates for the host when she stays with people.
But as well as the box for them, she takes a box for herself.
Lady Pamela said: "She does have her own box of chocolates, which she has learnt to keep in her room.
"Otherwise she says the family are so greedy they all eat them before she can - not our family, her family."
Lady Pamela shared a story one of the Queen's stays at Broadlands, the Mountbatten family home, when she had a pet mongoose, Naola.
She recalled: "One evening we were waiting for her to come down to dinner and it was very unusual for the Queen but she was four or five minutes late.
"She said to me 'I don't mind Noala coming into my room, in fact I quite enjoy it, I don't mind him opening my box of chocolates but does he have always to just take a bite out of every one?'"
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In the programme, which will be broadcast on ITV on Thursday at 9pm, she also reflected on being on the balcony after the royal wedding of 1947 as one of the then Princess Elizabeth's bridesmaids.
Lady Pamela said: "I was on cloud nine really. Tremendous excitement. It’s an extraordinary sight. There was an enormous crowd. They were a star couple. They were very much in love."
The programme features Lady Pamela talking through her royal life with her daughter, India Hicks, who reveals the true level of thriftiness in the Royal Family.
Hicks said: "I remember being handed me down Princess Anne's underwear - her chill-proof underwear.
"Now that's a long hand-me-down because there's quite an age gap between me and Anne. Imagine how many children wore that before it got to my little vest and bottom.
"That's a thrifty family."
Lady Pamela also revealed that the Queen once wrote to her mother to ask if she needed "quite so many new dresses" every year.
The Queen Mother gave her a "tart reply", Lady Pamela said.
Princess Anne was a bridesmaid for Lady Pamela Hicks, and India Hicks was bridesmaid for Prince Charles and Diana. Hicks is also Charles's goddaughter.
Lady Pamela said of her years with the Queen: "To have known The Queen, is to admire her above everybody else. She has always been accepting of whatever she is supposed to be doing. She deals with situations superbly and yet, of course, she is tremendously loyal and so, utterly selfless...
"It’s been an enormous privilege, watching her work and having fun with her, off duty. She is an amazing person. And she has remained like that throughout her life."
My Years With The Queen: Four Weddings and a Funeral…and a Coronation is on ITV1 at 9pm on 1 April.
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