British Airways on Sunday is resuming departures from London's Heathrow and Gatwick airports, following the cancellation of flights Saturday due to a worldwide computer outage that struck at the start of a busy holiday weekend.
"Work continues to restore all of our IT systems but we expect some further disruption today," British Airways posted on its Facebook page Sunday morning. "We are aiming to operate the majority of services from Heathrow and a near normal schedule at Gatwick. Customers should not travel to the airport today unless they have already rebooked onto another flight."
Heathrow echoed the airline's warning to travelers, tweeting that operations were not expected to be back to normal, tweeting Sunday morning that "delays and cancellations of British Airways flights are expected today."
And Gatwick tweeted, "Today, Sunday 28th May, British Airways are aiming to operate a near normal schedule at Gatwick."
British Airways had said a "major IT system failure" forced Saturday's cancellation of all scheduled flights from the two airports. The airline urged passengers booked on those flights not to go those airports.
The global outage also affected the airline's call centers.
"We are working hard to get our customers who were due to fly today onto the next available flights over the course of the rest of the weekend. Those unable to fly will be offered a full refund," British Airways said in a statement Saturday.
British Airways had said Saturday that it was working to restore services, but warned travelers that some delays and disruptions may continue into Sunday. Most "long-haul flights" set to land in London on Sunday are expected to arrive as scheduled, the airline said.
The outage comes amid a busy weekend for travel, with Monday being a holiday in both the United Kingdom and the United States.
"We are extremely sorry for the inconvenience this is causing our customers during this busy holiday period," British Airways said in its statement.
Heathrow Airport also confirmed the issue in a statement posted on Twitter.
ABC News on Saturday observed thousands of passengers at one of British Airways' terminals at Heathrow Airport, with some saying they were never alerted that their flights were cancelled.
ABC News' Christopher Donato, Matt Gutman, Dragana Jovanovic and Rex Sakamoto contributed to this report.