Frustrated travellers are facing major disruption at airports across the world after airline check-in systems crashed due to a "network issue".
Problems have been reported by passengers at London's Gatwick and Heathrow, and airports in New York, Australia, Paris, Washington, Singapore, South Korea and South Africa, among many other locations.
Travellers said they were unable to check in for their flights with a number of airlines - including British Airways, Air France, KLM, Qatar, Qantas, Lufthansa and Southwest - after the systems crashed, causing delays and long queues.
Travellers said they had been waiting to check in for nearly two hours without any luck, as the outage affected check-in services at airports and on internet browsers and mobile apps.
The problem involved Altea software used by more than 100 airlines, with the programme's creator, Amadeus, blaming the disruption on a network failure.
The issue affected tens of thousands of passengers and left many fuming.
Passengers faced disruption at Baltimore-Washington airport in the US (Image: MikeSchuh/Twitter)
The firm said its experts swiftly identified the problem and are "gradually" restoring services for the affected airlines.
The Altea software is used by 125 airlines around the world, with 108 using it for check-ins and boarding, aviation analyst Alex Macheras reported.
At the height of the outage, passengers around the world complained of problems checking in, making new bookings or updating existing reservations.
Passenger Tyrone Kennedy wrote on Twitter: "Systems down in Gatwick just hope it's not another #wannacry can't check in at the mo."
Another passenger wrote of Singapore's Changi airport: "Alert: Whole changi airport check in system down."
One Twitter user posted: "Awesome, system wide outage in Qantas. No idea when I can board the flight."
A Lufthansa passenger wrote: "Looks like @lufthansa’s computer system is down. Hope I can still get home today."
A Lufthansa spokesperson said the airline did not have to cancel any flights, although a few have been delayed.
Error messages greeted passengers as they tried to check in for flights (Image: Mike Schuh/Twitter)Passengers told of staff at airports, including OR Tambo in Johannesburg, South Africa, being forced to check in travellers manually.
Problems also affected passengers in the US.
One traveller wrote: "@SouthwestAir if your entire system is down at REAGAN airport in DC, does that mean the planes aren’t leaving on time?"
A passenger at Seoul's main airport wrote: "Korean Air’s system is down at ICN. Several planes being delayed. Doesn’t look good for @KoreanAir this Chuseok season."
The cause of the widespread outage was not clear.
It was also affecting flight buying systems used by travel agents.
IT firm Amadeus, the software's creator, said: “Amadeus confirms that during the morning, we experienced a network issue that caused disruption to some of our systems.
"As a result of the incident, customers experienced disruption to certain services.
"Amadeus technical teams took immediate action to identify the cause of the issue and restore services as quickly as possible. That action is ongoing with services gradually being restored.
"Amadeus regrets any inconvenience caused to customers.”
A spokesman for Gatwick said at 11am: "It was a momentary outage, affecting a few airlines, and everything is up and running as normal now."
KLM confirmed 24 flights from Schiphol have been delayed as a result of the problems.
A spokesman said: "There is currently a network failure in the Amadeus global computer system, which is used by KLM and other airlines.
"The failure has consequences for all Amadeus users, including KLM and Air France.
"Owing to the failure, the KLM booking system is temporarily not working as it should, and there also are problems with rebooking passengers.
"Previous problems relating to check-in have now been resolved. Until now, 24 flights from Schiphol have been delayed as a result."
But passengers continued to report problems elsewhere, with British Airways writing on Twitter: "We're aware of some system issues and we're treating it as high priority. We hope to have this resolved very soon."
At 12.30pm, a BA passenger tweeted: "I'm trying to book flights but it keeps coming up with an error message, phone line unavailable too... can anyone help pls?!"
Another added: "Hi, trying to get into my booking for my holiday to pay you money - comes up with an error all the time. can you help?"
Airlines at Heathrow's Terminals 2, 3 and 4 were affected.
A Heathrow spokesperson said: “A small number of airlines are currently experiencing intermittent issues with their check-in systems at airports around the world – including at Heathrow.
"Passengers will still be able to check-in for their flight, although the process may take slightly longer than usual.
"We are working closely with our airlines to help resolve the issue as quickly as possible. We apologise for any inconvenience this may cause.”
The situation left many passengers in the EU wondering if they would be entitled to compensation if their flights were cancelled or delayed by two hours or more.
Paloma Salmeron Planells, a spokeswoman for AirHelp, said: “Depending on the situation an IT glitch might not be regarded as an ‘extraordinary circumstance’, so if your flight is delayed or cancelled you could be entitled to up to £510 in compensation.
"In addition to this, the airline should provide you with meals, refreshments, access to phone calls and emails and if necessary, overnight accommodation."
The outage sparked fears of another global cyber attack before Amadeus announced that a "network issue" was at fault.
In May, the WannaCry ransomware cyber attack targeted computers around the world, with airlines, hospitals and universities among the victims.