Thousands of Delta Air Lines passengers around the world have been left stranded by flight cancellations, while many more face long waits after all of the company's departures were grounded over a system-wide computer failure.

"We lost power at about 2.30 this morning, which caused us to implemeng the ground stop," said the chief executive, Ed Bastian.

The carrier, one of the three biggest in the world, said the problem was a power cut at its Atlanta hub. Georgia Power, which supplies electricity to the Delta HQ, blamed an overnight failure involving switchgear. The outage lasted for six hours, ending at 8.40am, Atlanta time. By 1pm Delta had cancelled 427 flights, with many of its nearly 6,000 scheduled flights for the day heavily delayed.

The failure of the airline’s computer systems had only a minor effect on aircraft already in flight – disabling routine messages to and from the flight deck, without affecting safety. But prior to departure, vast amounts of data are exchanged between Delta’s Atlanta base and its teams at individual airports around the world. Check-in relies upon the airline's central reservations system, while messages about fuel, baggage and cargo must be sent before take-off. In addition, aircraft flying to the US are not allowed to depart until a full and complete passenger manifest has been supplied to the American authorities.

The problem hit at a key moment: the start of the working week, when the first wave of flights from Europe was due to depart to the US, along with evening departures from Asia.

Transatlantic passengers turning up at Heathrow Terminal Four for early departures to the US were among the first to be affected. For much of the morning, Delta flight 1 to New York JFK was shown on the airline’s website as “On time”. But that departure, like all the others worldwide, had been suspended. It was eventually cancelled.

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At Heathrow Terminal 4, confusion and frustration increased with little information forthcoming. Flights 59 to Boston and 195 to Philadelphia were delayed by four to five hours.

One passenger, Amanda Jackson, told NBC News that she waited more than 90 minutes to check in for a flight to Seattle on her way to Alaska. She claimed there were long lines at Delta counters, along with “a lot of very frustrated people.”

Passengers on Twitter also reported problems – including the inability to check in or being stuck on the Tarmac – from airports around the world, including San Francisco, Las Vegas and Athens.

As well as potential refunds, Delta has published a waiver for those travelling on Monday, allowing for changes to be booked without a fee.

Affected travellers may be able to claim compensation under EU passengers’ rights rules if they arrive at their destination three hours or more behind schedule, depending on whether the outage is regarded as an “extraordinary circumstance”.

All passengers are entitled to meals, and if necessary accommodation, until they can be flown to their destination.