The Duke of Cambridge will turn 39 on Monday, after appearing at a local running event with his two eldest children on Father’s Day.
William will be celebrating the final birthday of his 30s after surprising runners at a race around the Sandringham Estate on Sunday.
Pictures emerged showing William, Prince George and Princess Charlotte gathering around a microphone and appearing to count down to the start of the race.
George and Charlotte were photographed wearing shorts and trainers, while the duke was dressed in a blue shirt and a pair of navy chinos.
Brian Cook, who participated in the run and photographed the royal trio, said it was a “nice surprise, and nice of them to do”.
According to the event’s website, Run Sandringham was welcoming hundreds of runners of all abilities and ages to run a half marathon, 5K or community mile, with the aim of raising “many thousands of pounds for good causes”.
It is understood the duke was not running in the event and was there in a private capacity.
Prince William Arthur Philip Louis of Wales was born second in line to the throne at St Mary’s Hospital, Paddington, at 9.03pm on June 21 1982.
He weighed 7lb 1 1/2oz and was the first born son of the Prince and the Princess of Wales.
At just nine months old, he accompanied his parents on their six-week official visit to Australia and New Zealand, which was a break from royal tradition.
His mother Diana, who separated from Charles in 1992, died in a car crash in 1997.
He was just 15 and Harry 12, and the brothers walked behind the princess’s coffin during a funeral procession through London.
William met Kate Middleton at St Andrews University in Fife, Scotland, and the pair wed in 2011 after dating for more than eight years.
He was made the Duke of Cambridge by his grandmother the Queen on the morning of his wedding.
The couple welcomed their first child George – also a future king – in 2013, followed by Charlotte in 2015 and Louis in 2018.