Prince William and Kate Middleton have been caught out struggling to contain their giggles during the Royal Air Force Centenary service earlier today.
The Duke of Cambridge, 36, was spotted laughing after Prince Edward gestured at someone to kneel down at the Westminster Abbey service.
A camera caught a smirking William attempting to contain his laughs.
The Duchess of Cambridge, 36, catches the prince giggling, and she too attempts to contain her laughter.
Joanne Swinbourne, a lip-reading expert, told FEMAIL that Prince Edward had gestured at someone opposite him to kneel down.
She explained: 'Edward says to someone opposite, 'You can kneel if you want, kneel' and then gesturing to them to kneel.'
Later, the couple looked more serious as the service began.
The Duke and Duchess of Cambridge have had a busy week, christening their youngest son Prince Louis the day prior.