The Queen shares sweet birthday message for great grandson Prince Louis

Prince Louis is celebrating his 3rd birthday today. Photo / AP

After mourning the death of Prince Philip for the past few weeks, the royal family now has a reason to celebrate: young Prince Louis turning 3.

His parents Prince William and Kate Middleton shared a new photo of their youngest son to mark the occasion, showing him riding his red balance bike in a navy sweater and blue shorts.

And the Queen's official Instagram account reshared the new portrait of a grinning Prince Louis, shouldering a backpack on his way to his first day of preschool this week.

The Royal Family's page wrote, "Wishing Prince Louis a very happy birthday today," alongside a bike emoji.

Clarence House, the official account of Charles and Camilla, shared the photo to its stories as well, writing, "Wishing Prince Louis a very happy third birthday," along with a birthday cake emoji.

To celebrate Louis' 2nd birthday last year, his grandfather Charles shared a previously unseen snap of little Louis giving him a hug.

Prince Harry and Meghan Markle are yet to share any public birthday wishes for their nephew.

Louis, fifth in line to the throne, celebrates his birthday at the same time as starting at Willcocks Nursery School in London, the same preschool attended by his big sister Charlotte.

It's the perfect choice for Louis, since his older siblings Prince George, 7, and Princess Charlotte, 5, both go to Thomas's Battersea primary school close by.

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Kate has also previously shared snaps of Charlotte and George on their first days of nursery school. Charlotte started at Willcocks Nursery School in January 2018, while George went to Westacre Montessori School in Norfolk, starting in January 2016. Louis' later start date was probably the result of the latest UK lockdown amongst the ongoing Covid-19 pandemic.

It's the latest photo of several personal family snaps taken by Kate. The bike Louis is pictured riding is designed to help kids balance. It quickly sold out in England after the photo was published.

Sources told The Sun that William and Kate had chosen the Tadpole+ bike as it was a "great first bike to help your child to achieve the necessary balance to cycle," according to the company's website.