The royal family has wished a happy 21st birthday to Lady Louise Windsor, daughter of the Duke and Duchess of Edinburgh.

A message posted to the royal family’s official X account said: “Happy 21st Birthday to Lady Louise!”

The post was accompanied by a photo of Lady Louise at the Royal Windsor Horse Show this year and a birthday cake emoji.

Lady Louise – the youngest niece of the King and granddaughter to the late Queen – is studying English Literature at St Andrews University.

She is following in the footsteps of the Prince and Princess of Wales, who met at the university in Fife, Scotland in 2001.

William and Kate also reposted the royal family’s birthday message, adding: “Wishing Lady Louise a very Happy 21st Birthday”.

Louise was born prematurely in November 2003, weighing just 4lb 9oz.

Her mother, Sophie, was rushed to hospital at eight months pregnant, with severe internal pains and doctors found she was dangerously ill from blood loss and performed an emergency Caesarean.

Newborn Louise was transferred to St George’s Hospital in Tooting, London, for specialist care, while the duchess remained at Frimley Park Hospital near their Bagshot Park home. They were separated for six days.

The late Prince Philip taught Louise how to drive a carriage and she is often seen at the annual Royal Windsor Horse Show.

She occasionally accompanies her parents at events and attends royal celebrations such as Trooping the Colour.

She is 16th in line for the throne and she and her younger brother James, the Earl of Wessex, will be expected to work for a living and are unlikely to use HRH styles as adults.