Kate Middleton catches Prince William giggling during Westminster Abbey service

While waiting for the commencement of a ceremony held at Westminster Abbey to celebrate the 100th anniversary of the Royal Air Force, Prince William had a bit of a giggle, leaving everyone wondering what on earth the future King was laughing at?

In a video, the Duke of Cambridge can be seen sitting alongside wife Kate Middleton—who broke her maternity leave for a surprise appearance at the event.

Suddenly, a childish smirk creeps across Prince Will’s face before he bows his head and purses his lips in a failed attempt to hide the cheeky grin.

He appears to be on the verge of laughter when Kate—looking lavish in a pale teal Alexander McQueen dress and matching floral fascinator—catches him in the act.

As the camera pans out, the 36-year-old Prince can be seen glancing at his wrist as he explains the source of his smile to his wife, which leaves the duchess chuckling too.

While the origin of their chortles will remain a mystery, it was nice to see the royal couple and parents-of-three share an intimate moment under the roof of Westminster Abbey - the very place Kate and Will exchanged their wedding vows back in  2011.

Despite still being officially on maternity leave, Catherine attended the service as she is the Patron of the RAF Air Cadets, which was her first official Royal military appointment, taking over from the Duke of Edinburgh in 2015.

The event is a special one for Prince William too, who became a fourth generation RAF member and spent active duty as a search and rescue helicopter pilot.

Also attending the RAF Centenary event were Queen Elizabeth II, Prince Harry and Meghan Markle along with Prince Charles and Camilla.

In a sweet moment, Camilla helps a flustered Prince Charles with his gloves while Will and Kate laugh at the mix-up.

There were also plenty of loved up moments between Prince Harry and Meghan Markle who still can’t take their eyes off each other.