US airline giant Delta has suspended departing flights worldwide after suffering a major system outage.
“Customers should check the status of their flight before heading to the airport while the issue is being addressed,” the company tweeted.
It said it was working to resolve the problem as soon as possible. Inbound flights en route are not affected.
Delta ranks third in the world in terms of most passengers carried, according to industry body the IATA.
The problems were caused by an overnight power failure in Atlanta near Delta’s headquarters, the company said. It is not known how long they will take to resolve.
London’s Heathrow airport said: “Delta flights from Heathrow are experiencing delays due to the worldwide technical issue with their computer systems.”
“Check-in is currently operating using a back-up system and airport staff are on hand to assist any passengers that are impacted by the delays.”
One passenger told the BBC he was waiting with “several hundred” fellow stranded passengers at San Francisco airport, after being asked to leave a plane he had already boarded.
“We were ordered off the plane after approximately an hour or more,” said Dick Ginkowski.
“Tempers are starting to get just a little bit strained at this point,” he added. “They aren’t able to rebook anyone on other carriers.”
Delta has been responding to unhappy customers on Twitter.
Delta serves about 180 million customers a year, employing over 80,000 people, its website says. It began in the 1920s as a crop-dusting company.
The airline is the latest to suffer computer problems, with Southwest Airlines last month forced to cancel more than 2,000 flights after an outage prevented travellers checking in.