British Airways has cancelled all flights from Gatwick and Heathrow for the rest of Saturday after a global IT crash.
Passengers have been told not to travel to the London airports because of "extreme congestion", with scores left stranded at the terminals.
A spokeswoman for the airline said: "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. Those unable to fly will be offered a full refund."
BA said delays could continue into Sunday, but most long-haul flights should be able to land as normal in London.
"Following the major IT system failure experienced earlier today, with regret we have had to cancel all flights leaving from Heathrow and Gatwick for the rest of today, Saturday, May 27," the statement added.
"The system outage has also affected our call centres and our website but we will update customers as soon as we are able to.
"We are extremely sorry for the inconvenience this is causing our customers during this busy holiday period."
Scores of Britons were stranded as they tried to fly overseas for the long weekend and half-term school holidays, with many venting their frustration on BA's twitter page.
Several passengers at Heathrow told the Press Association they had not been told their flights were cancelled until more than an hour after the airline put out a press statement announcing the decision.
Images posted online showed a group of people gathered around the customer services desk at Terminal 5 trying to get information.
Terry Page, 28, from London, said: "There's no such announcement here. The boards are showing 'Go to gate' and no mention of cancellations."
More than an hour later, he said cancellations of individual flights were still being announced.
Footage filmed at the terminal showed long queues at customer services, after passengers had been advised that they would be unable to rebook due to systems remaining down.
Student Emily Wilson told the Press Association that she had been advised "we are unable to get bags, and that no more flights are taking off", several hours after having arrived at the airport for her flight to Stockholm.
Ms Wilson added: "We were told (it would be) about three hours for collecting bags, that all compensation will have to be done online, and that we are unable to rebook flights now because of the system being down."
She said that information on screens still suggested her flight could board shortly, but that staff contradicted that information, saying there were "no slots left".
Dan Power said he and his 80-year-old grandmother are stranded at Heathrow, waiting for a flight to Milan, and have had no information from BA.
He told BBC News they have been offered "absolutely nothing" in the way of refreshments and added: "We haven't been offered any chairs, any water, any vouchers - nothing.
"I don't think our week's holiday will happen at this rate.
"My main concern now is I don't want my 80-year-old grandma spending the night on Heathrow floor - but all the hotels are fully booked, we don't have any transport back up north, so we are actually stuck in Heathrow with nowhere to go."
Travellers have been told to check the airline's website and Twitter account for updates on the situation.
The airline has experienced issues with its online check-in systems in the past.
Passengers were hit by severe delays in September and July last year because of IT glitches.