Thousands of airline passengers whose flights have been cancelled due to the global IT outage are being urged to leave crowded airports.
Some 167 flights scheduled to depart UK airports have been axed, with others delayed, while 171 flights due to land in the UK were cancelled, following the technical glitch.
Passengers were forced to wait in long queues for check-in desks and to pass through security at airports such as Gatwick, Luton and Manchester.
Aviation analytics company Cirium said 5,078 flights – or 4.6% of those scheduled – were cancelled globally on Friday, including the 167 UK departures.
Ryanair, which said it was “forced to cancel a small number of flights today”, told passengers: “If your flight has been cancelled, we kindly request that you leave the airport as the IT outage means we cannot currently assist passengers at the airport.”
Edinburgh Airport said: “Anyone whose flight is cancelled is asked to please leave the airport and contact their airline directly.”
Transport Secretary Louise Haigh said her department was “working at pace with industry and across Government on the issue”.
A traveller at Gatwick Airport said he had been queuing for more than three-and-a-half hours ahead of a flight to Miami, Florida.
Dean Seddon, 42, from Plymouth, told the PA news agency: “There are just people everywhere, there must be 400 people in this queue for the check-in desk I’m at… it’s just bedlam.
Lance and Penny Spraggons said they had been waiting in a queue to check in at the West Sussex airport for more than an hour.
Mr Spraggons said: “The biggest problem is the lack of information.”
The queue at Stansted snaked outside the main terminal building.
Courtney Kemal, 32, who had been in the queue for around two hours by late morning, said her two sons aged five and seven were “obviously getting stressed”.
Rory Boland, editor of magazine Which? Travel, said passengers due to travel today “will naturally be deeply concerned”.
He went on: “If you can, avoid checking in a bag as queues for check-in at the airport will be long and IT failures typically lead to lots of lost bags.
“If you do check-in bags, make sure you keep medication, keys and any other essentials in your hand luggage.”
He said passengers will not be eligible for compensation for delayed or cancelled fights due to the “extraordinary circumstances”, but airlines have a duty to look after them, including providing meals, accommodation and alternative flight bookings if necessary.
UK air traffic control provider Nats said its systems are “operating normally”.
Friday is the busiest day of the year so far for UK air travel with 3,214 departing flights as thousands of families embark on summer holidays at the end of the academic year for many schools.
Several US carriers grounded flights, including United, American Airlines and Delta.
Train service information website National Rail Enquiries warned passengers of “widespread IT issues across the entire network”, although most train services ran as scheduled.
South Western Railway said all its ticket vending machines had stopped working due to IT issues.