Hong Kong airport authorities are investigating how five Singapore Airlines crew entered a restricted zone without security clearance last week, resulting in a flight bound for Singapore being delayed for close to three hours.

The flight attendants entered the departure level of the terminal without undergoing the necessary security checks, and there is suspicion a third party provided them with access.

The incident occurred on February 26 but only became public knowledge on Thursday.

The five crew members had arrived with Singapore Airlines flight 856, which landed in Hong Kong at 1.35pm. They were supposed to return to Changi Airport on the same plane, marked as flight 861, at 3.45pm that day.

The crew were either supposed to stay on the plane or carry their permits and exit it via the arrivals area, where they would undergo a security check before being allowed back on the aircraft. An alternative would have been to go through the transfer channel to access the departure gate.

However, the flight attendants exited the plane and went straight into the departure area of the terminal. They subsequently boarded the flight again.

Five minutes before the plane was due to take off, security officers called airport officials to say the crew had evaded screening. Police were then called.

The aircraft was not allowed to leave and all passengers were told to disembark and go through security clearance again.

A Hong Kong International Airport spokeswoman confirmed that five Singapore Airlines crew members "were found to have entered the restricted area departures level without carrying crew permits and undergoing security screening".

A Singapore Airlines source familiar with the matter said: "Our crew did something that wasn't right. They should know better. They shouldn't be going out via the departure level directly or from [the plane]. The door [that separates arriving and departing passengers] requires someone to swipe a card to open it.

"The crew problems inconvenienced passengers and that is why it is also being taken seriously by the airline," the source added.

The working theory is that when the attendants exited the plane, they walked up the jet bridge that passengers would use to board. A third party then opened a door controlled by a key card and code to allow them into the departure gate.

In a statement, a spokesman for Singapore Airlines said: "Flight SQ861 from Hong Kong to Singapore was delayed on February 26, 2018 as some of our cabin crew members operating the flight were required to assist airport authorities in an investigation. This resulted in additional security screenings, which required the passengers to disembark the aircraft.

"After the checks were completed, the crew members were allowed to operate the flight and passengers re-boarded, before the flight departed Hong Kong at around 6.20pm local time."