The Duke and Duchess of Cambridge have marked Prince Louis’s third birthday by releasing a photograph of their son pictured on his first day of nursery school.

Louis is growing up fast and is shown sitting on his bike in the grounds of Kensington Palace, smiling for mother Kate Middleton, who took the picture on Wednesday.

Wearing a jumper, shirt, shorts and with his rucksack on his back, the prince grips the handlebars and appears ready to cycle off to nearby Willcocks Nursery School – but was taken by car.

Louis celebrates his third birthday on Friday and is following in the footsteps of big sister Princess Charlotte, 5, who also attended the same nursery.

His birthday is likely to be celebrated with a cake shared with his new classmates – something revealed in the testimonials on the nursery’s website.

The duchess is a keen and accomplished amateur photographer, who in 2017 accepted a lifetime honorary membership of the Royal Photographic Society, which recognised her “talent and enthusiasm”.

She regularly releases images of her children to mark important occasions like birthdays and other major milestones.

Prince Louis as a baby in 2015, held by his great-grandmother, the Queen, in a photo released to mark the death of Prince Philip. Photo: Twitter/The Royal Family

The two-week period of royal mourning following the death of the Duke of Edinburgh ends on Friday, allowing the Cambridges to release the image.

The young prince was featured in a picture released in tribute to his great-grandfather, showing Philip and the Queen surrounded by their great grandchildren at Balmoral, with the monarch holding Louis.

The Cambridges’ youngest child is fifth in line to the throne.

-AAP