Tuilagi gave Cameron 'bunny ears' while stood behind and to the left of the Conservative Party leader as a group of players and management posed for a photo at the function to honour the Lions' 2-1 series victory over Australia.
You can see the incident in our video, and enjoy the end result in the picture here:
Using Twitter to express his remorse for the incident, the Leicester Tigers centre said: "Apologies for messing around on Lions photo. No offence intended. Great Day at Downing Street. Thanks to Prime Minister for hosting us."
And a brief apology on social media seems a small price to pay for the kudos that the player quickly earned from his fellow players, with fellow Lions star Ben Youngs saying on Twitter "The Man just couldn't stop himself" and Gloucester and England fly-half Freddie Burns tweeting: "Great bottle from Manu Tuilagi! Haha. #chief."
It's not the first time Tuilagi has courted controversy: at the 2011 Rugby World Cup he was fined £3,000 after jumping off a ferry in Auckland.