British Airways has cancelled all short-haul flights from Heathrow until midday because of technical issues.
The airline said the problem, which may also cause delays for its customers using Gatwick and London City airport, was related to a hardware issue and was not because of a cyber-attack.
The major outage has resulted in cancellations and delays of flights, pile-ups of luggage and passengers stuck on planes after landing at Heathrow.
BA said long-haul flights were still operating, but customers may experience some delays.
The airline’s website and app were inaccessible for hours on Friday evening, preventing customers from checking in online or booking flights.
On Saturday morning the airline advised customers to check the website for the latest flight information before coming to the airport.
The airline said: “We are extremely sorry that due to the continuing technical issues we are facing we have regrettably had to cancel all short-haul flights from Heathrow today until midday. Customers due to travel later today should check their flight status on ba.com before coming to the airport as we anticipate further disruption during the day.
“Our long-haul services at Heathrow and all flights at Gatwick and London City airport are due to operate as planned, but customers may experience some delays. Our website ba.com is working and customers can check-in online and at the airport.
“We are offering customers on cancelled services options including a full refund and all customers booked to travel on short-haul services from Heathrow today can opt to rebook to a later date for free if they choose. We will be contacting customers proactively.”
The airline said it would contact customers, adding: “Our teams have been working hard through the night and will continue to do so to resolve the issue as soon as possible. We advise our customers due to travel today to check ba.com for the latest flight information before coming to the airport.
“We know we have let our customers down and we will do everything we can to make this up to them – but for now our focus is on getting as many customers and flights away as we can.”
Photographs of departure boards in Heathrow Terminal 5 showed few flights boarding, while people complained on social media about a lack of information.
It is BA’s second outage in 10 days.