British Airways has cancelled all flights from Gatwick and Heathrow as computer problems cause disruption worldwide.
Passengers have been told not to travel to the London airports because of “extreme congestion” at the terminals, with all BA planes grounded before 6pm on Saturday.
A spokeswoman for the airline said: “We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide.”
“The terminals at Heathrow and Gatwick have become extremely congested and we have cancelled all flights from Heathrow and Gatwick before 6pm UK time today, so please do not come to the airports,” the airline continued.
“We are extremely sorry for the inconvenience this is causing our customers and we are working to resolve the situation as quickly as possible.”
Travellers have been told to check the airline website and twitter account for updates about the situation.
Heathrow Airport has been affected by the issue and advised passengers to check their flight status before travelling to the airport.
A spokesman said on Twitter: “We’re working with @British_Airways following an issue with their IT system this morning that has caused some delays for passengers.
“We advise passengers to check the status of their flight prior to travelling to the airport today.”
Teacher Gemma Richardson, 30, who is 24 weeks pregnant, was travelling with her husband David, 30, and their two-year-old daughter to Spain.
Mrs Richardson, who was sitting uncomfortably on a barrier, said “it was chaos” when they arrived at the airport.
She said: “There was no formal guidance, just information on a screen saying that the flights up to 6pm had been cancelled.
“We all pooled around a BA employee who was doing her best to give advice.
“It seems that because it is a bank holiday weekend there is no spare flights. We are on standby but it is very unlikely we are going anywhere.
“It is all stressful but we are trying to put a best face on it for my daughter but it is very difficult.
“There are no real chairs here so I am sitting on a barrier which is not great.”
Linda Murakami, 55, had flown in from her home in Pennsylvania, US, for a connecting flight to Naples, Italy, only to find it had been cancelled.
She is now cancelling a “dream holiday” to the Amalfi coast with her family.
Miss Murakami, who retired early from a pharmaceutical company, said: “This trip was my retirement present.
“There are a lot of other things that can cause disappointment and could put everyone in a worse situation.
“We just have to be flexible. We have travel insurance so feel that because we would miss part of the vacation, we might as cancel it and just go again at a different time. For me, it is an inconvenience.”
Her friend Joyce Cohen, of New Jersey, said: “This had been my dream vacation. Everything had been worked out so that I could come. I am very upset.
“I have waited for this my whole life and I am not in good health, so who knows what could happen by next year when I could come again.”
Malcolm Ginsberg, editor in chief at Business Travel News, said: “This is a very, very serious situation, one that will not be solved overnight, even once they get the technology aspects of it done - it’s going to be three or four days.
“There’s only full aircraft at this time of year and there will be aircraft in the wrong positions.
“Of course I feel sorry for the passengers but for the staff as well who have got to deal with it all.”