Cathay Pacific on Sunday said there had been an unauthorised boarding incident on the CX521 flight from Tokyo to Hong Kong last Thursday.

Hong Kong's flagship carrier said ground staff at Narita Airport immediately reported the incident and asked for help. Japanese police then took the alleged offender off the plane, detaining the person for investigation.

Cathay said the captain had then also conducted a comprehensive security check, during which all passengers were asked to leave the plane.

The flight arrived in Hong Kong at around 10pm on the same day.

The airline apologised for the incident, saying that the safety of passengers and crew is its top priority.