Civil aviation authorities are investigating how a child was able to pass through three checkpoints and board a plane at Beijing Capital International Airport without a ticket.

The incident happened on Sunday and was only discovered after flight attendants on Flight HO1252 from Beijing to Shanghai realized they had one extra passenger before takeoff.

The 4-year-old child had no boarding pass, although he was traveling with two adults and two older children, all of whom did have boarding passes, Beijing News reported on Tuesday.

A witness told the newspaper he heard one of the adults tell the flight crew they thought children shorter than 1.2 meters did not require tickets.

For safety reasons, all passengers were removed from the plane and a thorough security check of the aircraft was conducted, according to the report.

The flight was delayed and eventually took off at noon, more than five hours after its scheduled 6:55 am departure time.

After hearing about the incident, people have expressed concern about boarding procedures at the airport and civil aviation safety.

"That is a serious safety issue. Anything could have happened if the passenger was a terrorist," one netizen wrote.