Prince William back in the pilot seat as Kate celebrates her birthday

Prince William has spent his wife's birthday participating in training exercises at Royal London Hospital, flying with first responders in a rescue helicopter.

William worked as an air ambulance pilot before retiring in 2017 to become a full-time royal.

The Duke of Cambridge became Patron of their #30YearsSavingLives campaign which aims to highlight the charity’s vital work in delivering life-saving treatment across London, and raise support for the development of new facilities.

A video of the training exercises was shared on the young royal's official social media accounts, with the clip captioned, "The Duke of Cambridge, #30YearsSavingLives Patron, visited Royal London Hospital to learn more about the challenges @LDNairamb face in their life saving work as first responder in the capital."

However the prince made sure to properly mark his wife’s birthday.

During his appearance at Royal London Hospital Prince William was approached by two children who had made a card for the Duchess of Cambridge's 37th birthday.

He said, "Well done you for remembering," before adding, "I did remember this morning, so I was okay."

Prince William also praised the children, who are brother and sister, for their handwriting.

"Your writing is very good as well," he said.

It was later revealed the brother and sister were the children of Yair Shaahar who received care from the London Air Ambulance after a bicycle accident in 2016

It is thought Kate spent her birthday at home at Kensington Palace, with the Duke and Duchess of Cambridge celebrating with their three children Prince George, Prince Charlotte and Prince Louis.