David Cameron Instagram picture shows the PM asleep in awkward family photo posted by Emily Sheffield

(Instagram/Emily Sheffield)

It’s one of those awkward family photos that would be best gathering dust in an album, seen only by the eyes of one's nearest and dearest.

Unfortunately David Cameron’s sister-in-law published it to Instagram, where it could be viewed by millions.

The blushing bride-to-be Alice Sheffield, the half-sister of Samantha Cameron, is eclipsed by the snoozing man behind her: the Prime Minister

A barefoot Mr Cameron can be seen fast asleep next to his ministerial red box. Taken just after G20, it was clear being diplomatic with Mr Putin had taken its toll on the tired Tory.

It’s the same red box that got Mr Cameron into hot water earlier in the day, after he reportedly left it unattended while nipping off to the buffet car on a train journey.

A fellow passenger on the train took photograph of the well-worn red box on a train table with its key still in the lock.

"It was just sitting there. I could probably have run off with it if I'd wanted to," the passenger told the Daily Mirror.

Despite appearing dead to the world in this photo, Mr Cameron made it to his sister-in-law’s ceremony, where she tied the knot with her French estate agent fiancé, Etienne Cadestin, at the Sheffields' family home in North Yorkshire on 7 September.

The picture of the PM was posted on Instagram by Emily Sheffield, and has now been taken down.