The Queen was the first senior member of the Royal Family to send Kate Middleton 40th birthday wishes today.
In a post on Instagram, the monarch, 95, marked the milestone age of her grandson Prince William's wife with a series of throwback photos.
One shows Her Majesty with the Duchess of Cambridge on a joint engagement to Nottingham in 2012, while another is of them on the balcony of Buckingham Palace in 2016.
The caption of the carousel of images read: "Wishing The Duchess of Cambridge a very happy 40th birthday today!"
Royal fans have left over 178,000 'likes' on the post, with many also sending birthday messages to the duchess.
Similarly, Prince Charles and Camilla, Duchess of Cornwall shared their own celebration of the mother-of-three on Instagram this morning.
They posted an image of first-in-line to the throne Charles with his daughter-in-law at the Festival of Remembrance late last year.
It comes after she posed in Alexander McQueen dresses for three birthday portraits that will be displayed by the National Portrait Gallery.
In one, the duchess wears a pair of diamond and pearl drop earrings that once belonged to Princess Diana.
Her blue sapphire engagement ring, which was likewise worn by her late mother-in-law, is also visible in the elegant shot.
Posts on the duke and duchess' Instagram account sharing the images have already received millions of 'likes'.
Many rushed to comment that the photos were "stunning" and "beautiful", with several branding her "queen".
The royal birthday girl sat for the photos, taken by photographer Paolo Roversi, at Kew Gardens in November.
Members of the public will eventually be able to see the images on display in locations of significance to the duchess.
The caption to one of the posts read: "The three new portraits which have been released to mark the occasion will be shown in the community in three meaningful places: Berkshire, St Andrews, and Anglesey."
Berkshire is where the duchess grew up and where her parents still live, while St Andrews is where she met her husband, and the couple moved to Anglesey after their wedding in 2011.
Watch: Royal weddings through the years