A computer hardware problem forced United Airlines to ground all flights in the US for two hours on Wednesday morning.
The airline said 800 flights had been delayed, with four flights cancelled and 55 on its regional partners.
“We are in the process of restoring regular operations after a network connectivity issue impacted our systems,” United said yesterday afternoon.
“We apologise for the travel disruptions that our customers have experienced.”
United said it would rebook flights for affected passengers without charge.
The US Federal Aviation Administration said all flights operated by United were grounded from about 8am after the airline experienced a systemwide computer problem.
United flights were also grounded on June 2 due to “automation issues.”