Atlanta-based Delta, the second-largest U.S. airline by passenger traffic, said it had canceled 451 flights after a power outage that began around 2:30 a.m. EDT (0630 GMT) in Atlanta. Flights gradually resumed about six hours later.
The disruption could deal a blow to the reputation of Delta, which for months canceled far fewer flights than rivals and boasted of its successes to corporate and leisure customers to win their loyalty.
Delta said it was investigating the cause of the "system-wide outage." As of 1:30 p.m. EDT, it was operating about 1,679 of its nearly 6,000 scheduled flights.
The problems arose after a switchgear, which helps control and switch power flows like a circuit breaker in a home, malfunctioned for reasons that were not immediately clear, said Georgia Power spokesman John Kraft.
The utility, which provides electricity to most counties in Georgia, earlier sent a team to investigate, Kraft said. The problem has not affected other Georgia Power customers, he said.
The carrier was probably running a routine test of its backup power supplies when the switchgear failed and locked Delta out of its reserve generators as well as from Georgia Power, industry analyst and former airline executive Robert Mann said. That would result in a shutdown of Delta's data center, which controls bookings, flight operations and other critical systems, he said.
A Delta spokeswoman declined to comment when asked about backup systems.
Industry consultants say airlines face an increasing risk from computer disruptions as they automate more of their operations, distribute boarding passes on smartphones and outfit their planes with Wi-Fi.
Delta's flight information was not showing correctly on Delta's website or on airport information boards, and this could also take time to resolve, the carrier said. Mann said monitors typically display cached data until the computer system updates with new information.
According to website Flightradar24, some of the first flights to take off were from Amsterdam to the United States, while a flight from Phoenix to Atlanta was among the first to depart from a U.S. airport.
Delta is a member of the SkyTeam alliance alongside airlines including Air France-KLM SA. It also partners for transatlantic flights with Virgin Atlantic Airways Ltd, which said its flights were operating normally but cautioned that passengers should check tickets in case their flight was due to be operated by Delta as part of a code-share agreement.
Delta said passengers booked for travel Aug. 8-12 would be entitled to a refund if their flight is canceled or significantly delayed.
Delta shares were down less than 1 percent in afternoon trading.
In airports around the world, passengers stuck in check-in queues or on planes waiting to depart took to Twitter to share photos and frustration at the delays, as well as to ask how a major airline could be grounded by a power cut.
Delta's outage follows several high-profile computer problems faced by U.S. airlines in the past year.
Budget carrier Southwest Airlines Co halted departures last month after a technical problem, while American Airlines Group Inc suspended flights from three of its hubs last September after technical problems.