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.
"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."
Camilla also marvels at the Queen's stamina as she continues to ride out most weekends.
"Who else is riding at 90? That's what's so incredible," says Camilla.
But even the Queen has her limits, it seems. The documentary shows Her Majesty being told by an aide that the weather forecast doesn't favour riding, prompting the Queen to admit: "It's quite difficult at this time of year... and I'm rather a fair-weather rider at the moment. I don't like getting cold and wet."
To make the programme, TV cameras were given access to the Queen's life for a year, from attending public events to simply relaxing at home.
She will celebrate her birthday on April 21 at Windsor Castle, but there are events around the UK until June to mark the occasion - the first time a reigning monarch has turned 90.
Camilla is joined by other members of the Royal family contributing to Our Queen At 90, which will be broadcast tonight on ITV 8pm Easter Sunday (UK time).
The Duchess of Cambridge tells during the two-hour programme how Prince George, two, affectionately calls his great-grandmother "Gan-Gan" and reveals that she once gave the Queen a jar of home-made chutney for Christmas.
World leaders congratulating the Queen in the documentary include US President Barack Obama, who describes her as "a source of strength and inspiration".
- Daily Mail