Tens of thousands of British Airways' customers were left stranded on Saturday after the airline was forced to cancel all of its scheduled flights from London's Heathrow and Gatwick airports due to a computer breakdown.
We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide the airline said in a statement.
At first, the airline announced that it was canceling all flights until 6.00 pm (1700 GMT) from Heathrow and Gatwick today, a spokesman said.
But later, it admitted the disruption would last for the rest of the day.
On its website, BA said it expected London-bound long-haul flights to land as scheduled on Sunday, but that thousands of travelers from the two airports may continue to face delays due to staff and aircraft being in the wrong location.
The company later said it was aiming to operate a near normal schedule at Gatwick and the majority of services from Heathrow on Sunday.
In a video message to affected customers, BA's Chairman and CEO Alex Cruz said the airline would make arrangements to get customers on their way as soon as we possibly can.
Some British media speculated the system failure was the result of a cyberattack but BA rejected that claim.
The airline said it had found no evidence that it's a cyberattack, with Britain still recovering from a the Manchester attack that crippled key infrastructure earlier this month.
The system failure is causing massive delays with planes stuck on runways, and the terminals at London's two major airports getting extremely crowded.
On Sunday, the airline said that it would aim to resume flights from Gatwick as well as the "majority" of services from Heathrow.
It also said it would refund those who suffered cancellations and no longer wanted to fly to and from the affected airports.
Although some of the IT systems have returned, there will be some knock-on disruption to our schedules as aircraft and crews are out of position around the world, the airline said in a statement on its Facebook page Sunday.
It asked travelers not to go to Gatwick or Heathrow unless they are confirmed for travel.
We are repositioning some aircraft during the night to enable us to operate as much of our schedule as possible throughout Sunday, the airline said.
At this stage we are aiming to operate a near normal schedule of flights from Gatwick and the majority of our Heathrow services.
British Airways has extended its flexible booking policy and says customers can rebook their flights to travel up to and including June 10.
On Saturday, British Airways CEO Alex Cruz posted a video on Twitter in which he said he believed the cause of the problem to be a power supply issue.
He also rejected any notion that the airline had been the subject of a cyberattack.
The airline had initially canceled only flights before 1 p.m. ET (6 p.m. BST) at Heathrow and Gatwick.
Travelers in the United Kingdom were already reporting disruptions, including baggage and flight delays, when the airline reported serious computer problems around midday Saturday.
A Nigerian passenger visiting his wife in the United Kingdom on their wedding anniversary, said his plane sat on the tarmac at Heathrow for about four hours after landing. The plane's entertainment system and air conditioning were operating, and free snacks and drinks were served.
Most travelers seemed resigned, he said, though others appeared stressed and some children were crying.
Most people were being rebooked on Sunday flights, he said, but by then, he'd already have missed his match.
BA staff were slow on announcements and updates, and were bringing out cordons and bottles of water to manage queues.
They seemed incapable of doing anything else, despite their best intentions, due to the system outage. They were totally crippled.
Passengers at London's Heathrow Airport reported flight delays and long queues at check-in counters.
BA said the crash also affected its website, mobile app and call centers and told passengers not to travel to the two airports.
Meanwhile, some frustrated customers took to Twitter to voice their complaints.
Still on the tarmac at Leeds. #Britishairways reckon Heathrow is so backed up we can't set off. No way we'll make our Vegas flight, one passenger wrote on Twitter.
The disruption comes at the start of a three-day weekend in Britain.
London's Heathrow Airport, one of the world's busiest, said it was working with BA.
Passenger Roshni Burt, who was flying from Heathrow to Bahrain with her young son, said there was no news about when her flight would depart.
When we left the check-in area there were angry people, people getting frustrated that their flights were coming up or near to departure, people getting turned away with BA staff basically saying, If you've not checked in online, you've missed your flight, she said.
Computer glitches and outages have plagued BA and other airlines in before.
In September, British Airways said a computer problem in its check-in systems caused delays and long lines at numerous airports.
Delta was hit by a global computer outage one month earlier that caused days of travel chaos, including about 2,000 flight cancellations.
And in July, Southwest Airlines canceled more than 1,000 flights following a system failure.