LONDON -- British Airways (BA) said late Saturday that they aim to have normal operation at Gatwick airport and close to normal at Heathrow running on Sunday, after a day of chaos caused by a "very severe disruption" to its IT system.

The airline had canceled all flights from London's Heathrow and Gatwick airports Saturday, the first day of a bank holiday weekend. The airline advised people to only come to the airports Sunday if they have a confirmed reservation, and warned there still could be disruptions.

The airline told CBS News correspondent Kris Van Cleave it has no evidence the disruption is the result of a cyber attack.

Get Breaking News Delivered to Your Inbox

CEO Alex Cruz said in a video statement released Saturday that the airline will issue full refunds to passengers who choose not to travel because of the outage.

"We believe the root cause was a power-supply issue and we have no evidence of any cyberattack," Cruz said.

BA operates hundreds of flights from the two London airports on a typical day -- and both are major hubs for worldwide travel.

Earlier, BA had canceled flights from the two airports until 6 p.m. local time. Heathrow airport said later Saturday that all flights were canceled for the rest of the day.

"We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide," the company said in a statement. "The terminals at Heathrow and Gatwick have become extremely congested ... so please do not come to the airports."

BA says it is "extremely sorry for the inconvenience," adding it is "working to resolve the situation as quickly as possible."

Heathrow said it was providing meal vouchers, water and snacks to passengers stranded at the airport. It advised passengers booked on flights scheduled to leave Sunday morning to check the status of their flight ahead of their scheduled departure time.

Stranded travelers sit with their luggage outside the departure area of Heathrow Airport Terminal 5 after British Airways flights were cancelled are seen at Heathrow Airport in west London on May 27, 2017. Daniel Leal-olivas/AFP/Getty Images

The problem comes on a bank holiday weekend, when tens of thousands of Britons and their families are traveling.

Passengers at Heathrow reported long lines at check-in counters and the failure of both the airline's website and its mobile app. BA said the crash also affected its call centers.

Passenger Phillip Norton tweeted video of an announcement from a pilot to passengers at Rome's Fiumicino airport, saying the problem affects the system that regulates what passengers and baggage go on which aircraft. The pilot said passengers on planes that have landed at Heathrow were unable to get off because there was nowhere to park.

One person posted a picture on Twitter of BA staff writing gate numbers on a white board.

"We've tried all of the self-check-in machines. None were working, apart from one," said Terry Page, booked on a flight to Texas. "There was a huge queue for it and it later transpired that it didn't actually work, but you didn't discover that until you got to the front."

Another traveler, PR executive Melissa Davis, said her BA plane was held for more than 90 minutes on the tarmac at Heathrow on a flight arriving from Belfast.

A mass of delays (although not showing in red) terminal 5 #britishairways pic.twitter.com/etqo5sNdBK — Martin Patience (@martinpatience) May 27, 2017

She said passengers had been told they could not transfer to other flights because "they can't bring up our details."

Some BA flights were still arriving at Heathrow on Saturday, although with delays.

American Airlines, which operates code-share flights with BA, said it was unaffected.

Air industry consultant John Strickland said Saturday's problems would have "a massive knock-on effect" for several days.

"Manpower, dealing with the backlog of aircraft out of position, parking spaces for the aircraft -- it's a challenge and a choreographic nightmare," he said.

Airlines depend on huge, overlapping and complex IT systems to do just about everything, from operating flights to handling ticketing, boarding, websites and mobile-phone apps. Some critics say complex airline technology systems have not always kept up with the times.

Travelers wait at Heathrow Airport Terminal 5 after British Airways flights where cancelled at Heathrow Airport in west London on May 27, 2017. Daniel Leal-olivas/AFP/Getty Images

And after years of rapid consolidation in the business, these computer systems may be a hodgepodge of parts of varying ages and from different merger partners, all layered on top of each other.

"This could have all been avoided," said Mick Rix, national officer for aviation at the GMB union.

While not that frequent, when airline outages do happen, the effects are widespread, high-profile and can hit travelers across the globe.

BA passengers were hit with severe delays in July and September 2016 because of problems with the airline's online check-in systems.

In August 2016, Delta planes around the world were grounded when an electrical component failed and led to a shutdown of the transformer that provides power to the airline's data center. While the system moved to backup power, not all of the servers were connected to that source, which caused the cascading problem.

Delta said it lost $100 million in revenue as a result of the outage. In January, it suffered another glitch that grounded flights in the U.S. That same month, United also grounded flights because of a computer problem.

In July, meanwhile, Southwest Airlines canceled more than 2,000 flights after an outage that it blamed on a failed network router.

After the recent outages, outside experts have questioned whether airlines have enough redundancy in their huge, complex IT systems and test them frequently enough.