Serious problems with British Airways’ IT systems have led to thousands of passengers having their plans disrupted, after all flights from Heathrow and Gatwick were cancelled.
Passengers described “chaotic” scenes at the airports, with some criticising BA for a lack of information.
The airline has apologised, and told passengers not to come to the airport.
BA chief executive Alex Cruz said: “We believe the root cause was a power supply issue.”
In a video statement released via Twitter, he added: “I am really sorry we don’t have better news as yet, but I can assure you our teams are working as hard as they can to resolve these issues.”
Mr Cruz said there was no evidence the computer problems were the result of a cyber attack.
The airline hoped to be able to operate some long haul inbound flights on Saturday, landing in London on Sunday, Mr Cruz added.
The GMB union has suggested the failure could have been avoided, had the airline not outsourced its IT work.
BA denied the claim, saying: “We would never compromise the integrity and security of our IT systems”.
All passengers affected by the failure – which coincides with the first weekend of the half-term holiday for many in the UK – will be offered the option of rescheduling or a refund.
The airline, which had previously said flights would be cancelled until 18:00 BST, has now cancelled all flights for Saturday and asked passengers not to come to Gatwick or Heathrow airports.
Other airlines flying in and out of the two airports are unaffected.
Architect and TV presenter George Clarke was stuck in Heathrow. He told the BBC it was one of the “most turbulent, badly organised days, that I’ve ever experienced in Britain”.
“The lack of communication all day was woeful. There wasn’t a single Tannoy announcement all day in the terminal, not a single member of staff came up to us,” he said.
“The only time I found out my flight was cancelled was from the BBC News website.”