British Airways has canceled all Saturday flights from London’s two biggest airports after what it called “a major IT system failure.”

“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,” the airline said. Airline officials have no evidence of a cyberattack, they’ve said. British Airways has canceled all flights from London’s two biggest airports for at least several hours after experiencing “a major IT system failure that is causing very severe disruption to our flight operations worldwide,” the airline said Saturday.