British Airways cancelled all its flights out of major London airports Heathrow and Gatwick on Saturday after an IT systems failure, leaving hundreds stranded on a busy holiday weekend.
"We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide," BA said in an early afternoon statement.
BA flights out of Heathrow and Gatwick were initially cancelled until 6.00 pm UK time (1700 GMT) but the airline later said there would be no flights for the rest of the day.
It said it had found "no evidence that it's a cyberattack", with Britain still recovering from a ransomware attack that crippled key infrastructure earlier this month.
"We are working hard to get our customers who were due to fly today onto the next available flights over the course of the rest of the weekend," BA said, adding that its call centres and website were also affected.
The outage comes on a busy holiday weekend in Britain, where Monday is a public holiday and many schoolchildren are beginning their half-term break.
BA warned people not to come to the "extremely congested" airports, but an AFP photographer at Heathrow's Terminal 5 said many travellers were waiting outside the departure area with their luggage in "chaotic" scenes.
They were not being allowed inside the terminal for health and safety reasons, prompting some of them to take taxis away from the transport hub.
Travellers wrote on Twitter that they were unable to check in or use the airline's app, while others said they had been left stranded on the tarmac.
"None of your staff know what the heck is going on. Nor do your passengers. Can we have some info please? Chaos here," tweeted Chris Ship, Royal Editor for ITV News, to BA.
One passenger, Julie Adie, wrote that she had been stuck on the runway for an hour without an offer of drinks.
Another Twitter user called Jo called it a "terrorist's dream", writing that "people getting off plane with/without luggage. Some staying on".
BA said that those unable to fly would be offered a full refund, warning that some delays and disruption could continue into Sunday.
"Most long-haul flights due to land in London tomorrow are expected to arrive as normal, and we are working to restore our services from tomorrow," the company said.
Delays were reported in other European cities including Rome, Prague, Milan, Stockholm and Malaga.
Inside Heathrow's Terminal 5, Luke Hallard wrote that "everyone remarkably calm despite British Airways chaos, but wait until the lounge runs out of booze".
Alma Saffari told the BBC that her flight from Marseille to London had been grounded.
"When we finally boarded the captain came out and told us their computer systems were down worldwide," she said.
"Eventually after sitting on the tarmac for one and a half hours we disembarked the plane.
"Now we are sitting in the departure area outside the gate."
The company said it was "extremely sorry for the inconvenience this is causing our customers during this busy holiday period".
The airline has suffered other IT glitches recently, leading to severe delays for passengers in July and September last year.