“Certainly, I did not expect this would happen to an international airport,” said 27-year-old business traveller Peter Lapointe heading to Vietnam.

The Airport Authority apologised for the inconvenience to passengers and said only “a few” flights suffered delays, but admitted fixing the problem took longer than expected. The government told the authority to investigate and file a report.

Authority executive director Steven Yiu Siu-chung insisted airport operations were able to continue as normal, but stopped short of saying when the system would be fully restored.

“Basically, the airport operation has been normal. Travellers are able to check in and board the aircraft. The security checks are not affected as well,” Yiu said, noting that the authority handled 360 flights between 7am and 3pm. “No passengers had their flights cancelled or delayed due to the system failure.”

Passengers first noticed the massive overhead screens in the terminal failed to update at around 7am, but the authority only alerted the public to the computer breakdown two hours later.

The authority’s MyHKG mobile app and website also displayed “flight information currently unavailable” because of technical problems. The baggage reclaim hall in the airport’s restricted area also failed to display information for passengers.

The Airport Emergency Centre was activated and implemented contingency measures, including setting up temporary screens to display information, after confirming that the system could not be restored within a short period of time in the morning.

Notices were attached to each display screen in the departure hall to tell passengers about the update failure, with the same information broadcast every 15 minutes over the public address system.

Display screens at the boarding gate also failed to update the latest flight information, and airline staff were seen holding handwritten information in restricted areas to tell passengers about gate changes.

But Macau resident Jacob Ng, 40, said he missed his flight to Cebu in the Philippines and was unsure what to do as the airline only flew there once on a Sunday.

Ng, a manufacturer, said he arrived at the airport just after midnight for his 9.40am flight and entered the restricted area at 6.30am with a boarding pass that showed no gate information.

“After I entered the restricted area, I didn’t find any info about my flight until around 10am when I was told where the gate was,” he said.

Arriving at the gate, he found he was one of six passengers who missed the flight.

“I am disappointed with Hong Kong International Airport. How can their risk management be qualified to tell good Hong Kong stories?”

As of 10pm on Sunday, the gate numbers for a handful of flights over the next three hours were still missing from the display boards in the departure hall and on flight tickets printed at self check-in kiosks.

The airport website showed upcoming flights’ information but did not specify gate numbers, while the mobile app adopted a temporary layout to display boarding details. although crucial information was still missing.

Ricky Leung Wing-kee, an executive director of engineering and technology at the authority, said no hacking of the airport’s computers had been detected, and there would be “thorough checking” of the system maintained by in-house personnel.

Information security management specialist Ronald Pong said he believed the system failure arose from the authority’s backend system, urging it to look into the issue to prevent similar incidents from happening again.

He also described the authority’s response time as “unacceptable” as the airport only disclosed the incident two hours after abnormalities were spotted, which created “a lot of unnecessary hurdles for passengers”.

Lawmaker Ben Chan Han-pan, who chairs the Legislative Council’s transport panel, said handwritten flight information appeared to be the only available option, although it was not ideal.

“The Airport Authority should have a backup system. It’s not clear whether the backup system was activated or whether it could help specifically for flight information,” Chan said.

The lawmaker said that, although computer glitches were unavoidable, the authority needed to keep up with regular inspections, testing and drills.

Secretary for Transport and Logistics Lam Sai-hung said the authority was required to submit an investigation report to the government about the incident.

“I have requested the authority to disseminate the latest information to travellers in a timely manner, investigate the cause of the incident and report to the Transport and Logistics Bureau,” he said in a Facebook post.

The MTR Corporation said pre-check-in services for the Airport Express at Hong Kong station and Kowloon station were operating as usual, but it was not able to provide flight information.

It also raised questions about the airport’s ability to effectively handle emergencies.