Pakistan International Airlines (PIA) are investigating how ground crew failed to notice the woman during the plane's two-hour stopover at Charles de Gaulle Airport in Paris.

The woman, named as Patrice Christine Ahmed, who is married to a Pakistani, left Lahore at noon on Tuesday to fly to Paris via Milan, but did not wake up to get off the plane, airline spokesman Sultan Hasan said.

The woman did not mention her mistake to cabin crew and the matter only came to light when she was stopped by immigration officials on arrival back in Lahore on Wednesday morning - after a 12,000-kilometre round trip.

Hasan said PIA were investigating the incident and the French subcontractor responsible for passenger handling in Paris.

"We have put questions to this French firm also about the incident but it is also the responsibility of the passenger to disembark at the destination," he said.

"It is a passenger's responsibility to check about the destination and disembark when the plane arrives at the particular airport."

PIA later arranged to send the woman back to Paris with another airline because none of its own flights were available, but said that the party responsible for the negligence will pay for the extra ticket.

"It depends who is at fault. If it is a mistake by the local firm, they will pay and if the woman herself is responsible than she will have to bear the cost," Hasan said.