harry and william image

Prince William shares adorable throwback picture featuring Prince Harry and Princess Diana

Take a look at the cute photo below...

Sharnaz Shahid,

The Duke of Cambridge has marked 999 Day in the UK in a very special way. On Monday, Kensington Palace released a series of throwback pictures of the royal - including a childhood photo featuring William with his brother Prince Harry and their late mother, Princess Diana, as they posed with a vintage fire engine at the Old Fire Station. The snap, which was taken in 1988, also included the royal brothers' cousins Peter Phillips and Zara Tindall.

Prince William posted this throwback snap

999 Day promotes the work and efforts of the emergency services as well as volunteers, who play a core part in keeping Britain safe. In the caption, Kensington Palace wrote: "Today marks #999DayUK, when we celebrate and say thank you to our incredible Police, Fire and Emergency Medical Services and all our specialised emergency services for all that they do to keep us safe."

MORE: Prince William and Kate reach incredible social media milestone

Drawing attention to the old photo, the message continued: "The first photograph in this post is of The Duke of Cambridge and The Duke of Sussex and their mother Diana, Princess of Wales, with their cousins Peter Phillips and Zara Tindall in the yard of the Old Fire Station in the grounds of Sandringham House in 1988."

WATCH: Meghan Markle surprises fans with royal wave during visit to New York

Prince William, who worked as an air ambulance pilot himself, will later visit The Fire fighters Charity at Harcombe House in Devon to hear about the support that the centre provides to members of the fire and rescue service and their families, as well as meet with representatives from across the emergency services sector.

Ahead of the engagement, William said in a statement: "As a society, we owe our wellbeing, and indeed our lives, to the men and women in our emergency services who work tirelessly to protect us in some of the most difficult circumstances." He added: "It is only fitting that we should recognise the vital role that they play, and pay tribute to the bravery and dedication of those who have made the ultimate sacrifice for their communities."

Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity, royal and lifestyle news delivered directly to your inbox.