Prince William struggled to contain his giggles at the start of the RAF Centenary service today - as a cool Kate managed to keep her composure.
The Duke of Cambridge, 36, was seen laughing after Prince Edward gestured at someone to kneel down at Westminster Abbey.
The camera then cut back to a smirking William, who was still trying to hold in his laughter.
The Duchess of Cambridge, 36, is then seen catching him giggling, with the pair exchanging a smile.
William then appears to tell her when the service is expected to start, gesturing to his watch.
Lip-reading expert Joanne Swinbourne told FEMAIL that Edward, 54, appeared to be gesturing at someone opposite to kneel down.
She explained: 'Edward says to someone opposite, “You can kneel if you want, kneel“ gesturing to them to kneel.'
Both William and Kate can seen be laughing at his remarks, before the camera cuts away to Prince Harry and Meghan.
However, when the camera returns to the couple, William is seen struggling to contain his laughter.
Later on, the Duke was seen looking more serious as the service at Westminster Abbey began.
William and brother Harry, 33, attended the service in their RAF uniforms today, while Kate was also appropriately dressed in blue for the Centenary celebrations.
The mother-of-three opted for a tried and tested style as she wore the same Alexander McQueen coat worn to Harry and Meghan's wedding, but this time in a pale blue colour.
As the Honorary Air Commandant of the RAF Kate could be seen wearing Air Cadets Dacre brooch, known to be the highest award any female cadet can achieve as she arrived at Westminster Abbey for the service.
It has been a busy week for the Cambridges, who attended Prince Louis' christening at the Chapel Royal at St James's Palace on Monday.
The guest list for the christening included the Prince of Wales and The Duchess of Cornwall, along with Harry and Meghan and the Middleton family.
The Queen and Prince Philip were notably absent at the christening.
This article has been adapted from its original source.
