They haven't always enjoyed the warmest of relationships but in a landmark television documentary to be aired in the UK on Easter Sunday, the Duchess of Cornwall will give an intimate insight into her increasingly close friendship with the Queen.
In a rare interview celebrating the monarch's 90th birthday, Camilla fondly refers to her mother-in-law as "the tops" - and praises the Queen's unrivalled knowledge of the equestrian world.
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"If I need to ask about a horse there's no better person to ask, I mean she is the tops. You couldn't find a better expert," says Camilla.
"Horses respond to her. It's a two-way relationship, they both love each other. She has that wonderful smile which lights up her whole face and you can see the real joy she gets from it."