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."There's no evidence it's a cyberattack," airline officials said.Flights before 1 p.m. ET (6 p.m. BST) were canceled, with more cancellations possible."The terminals at Heathrow and Gatwick have become extremely congested and we have cancelled all flights from Heathrow and Gatwick before 6pm UK time today, so please do not come to the airports," British Airways said in a statement."We will provide more information on ba.com, Twitter and through airport communication channels as soon as we can for flights due to depart after that time."The airline has not said how many flights will be delayed or canceled, nor how many people are likely to be affected.In response to tweets from travelers, it said that it was experiencing a "global system outage" which was affecting its website, online check-in, contact centers and baggage tracing."Working on this as a matter of priority, but if your flight is departing tomorrow you may have to complete check in at the airport," it tweeted to one customer.The airline said it was "extremely sorry" for passengers' inconvenience and was working to resolve the problem as soon as possible. Airline officials gave no indication of what might have caused the problemor when it would be fixed.