Nancy Cameron was recently left alone at the Plough Inn, in Cadsden in southeast England, for around 15 minutes, the BBC reported.
According to a Downing Street statement, the Cameron family ate Sunday lunch there with friends before returning to their nearby country residence in two separate cars.
David Cameron assumed Nancy was with his wife, Samantha, and two other children in the second car; while they thought she was with her father and his bodyguards.
The Camerons were "distraught" when they realized the mistake, according to the prime minister's spokesperson, and immediately returned to the Plough in search of Nancy, where they found her with pub staff.
The Sun quoted an unnamed pub "insider" as saying employees had come across Nancy in the ladies' bathroom.
"It's frightening the prime minister of Britain can forget something so important as his own daughter," the source told the tabloid.
Downing Street declined to comment on whether Cameron had drunk alcohol with his lunch, the Guardian said.
Commentators on the BBC's website were divided over the incident, some charging Cameron with irresponsibility and others describing it as a simple mistake.
"Some people will use this to take a swipe at Cameron – most will see the funny side and admit that it could happen to anyone if the family is using more than one car," wrote one.
The mix-up, which is said to have taken place a couple of months ago, has come to light at an embarrassing moment for the prime minister, whose government today launched a program to tackle "problem families," the Guardian noted.