LONDON (Reuters) - Prime Minister David Cameron accidentally left his daughter behind in a country pub after a Sunday lunch with friends following a mix-up over which car she was meant to be going home in, his Downing Street office said on Monday.

Cameron was swiftly reunited with 8-year-old Nancy, one of his three young children, but the incident will add fuel to critics who accuse him of being overly fond of relaxing, or "chillaxing" as he has called it, when not dealing with affairs of state.

Its disclosure in the Sun newspaper has the potential to embarrass Cameron, coming on the day the government launched a fresh drive to tackle "problem families" who lead chaotic lives and cost taxpayers millions of pounds in policing and welfare costs.

Cameron and his wife Samantha only discovered their eldest child was missing when they returned to their official country residence, Chequers in Buckinghamshire, 40 miles northwest of London.

The 16th century mansion is a short drive from the Plough Inn where they had been dining with two other families.

Nancy had gone to the toilet and been overlooked when her parents left in separate vehicles.

"The Prime Minister and Samantha were distraught when they realized Nancy wasn't with them," said a Downing Street spokeswoman, adding that the incident occurred around two months ago.

"Thankfully when they phoned the pub, she was there safe and well. The Prime Minister went down straight to get her," she added.

(Reporting by Tim Castle; Editing by Steve Addison)