The couple’s daughter Nancy wandered off to the toilets while they were arranging lifts and they only realised she was not with them when they got home, the Sun reported on Monday.
Mr Cameron rushed back to the Plough Inn in Cadsden, Bucks, where he found his daughter with staff.
A Downing Street spokesman said: “The prime minister and Samantha were distraught when they realised Nancy wasn’t with them.
“Thankfully when they phoned the pub she was there safe and well.
“The prime minister went down straight away to get her.”
The Camerons were at the Plough Inn – near the prime minister’s country retreat Chequers – with Nancy and their other children Arthur, six, and 22-month-old Florence, as well as two other families.
When Mr Cameron left the pub, he went home in one car with his bodyguards and thought Nancy was with his wife and their other children in another car.
Mrs Cameron had assumed her eldest daughter was with her father.
The mistake was only discovered when they got home.
Mr Cameron drove back to the pub and found Nancy helping staff.
She was away from her parents for about 15 minutes.
Downing Street said the incident happened “a couple of months ago”.
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Your tweets: @27reasonstorock Hubby left our baby in airport security had to be called back "would the gent who left his baby with us please reclaim her."! @andreemurphy reckon if it had been me, mother of 5 from Belfast, it wouldnt be funny tale swapping but calling social services... @markie37 my security staff always did a head count after a few beers in the pub in case of kidnapping etc.