A “major IT system failure” is causing British Airways some headaches today. The airline reports that the computer issues are causing problems worldwide, and that flights out of London’s Heathrow and Gatwick airports have been canceled.

The airline is advising passengers to avoid going to the airport if they have a flight schedule before 6PM, to mitigate already crowded terminals, and that it’s unable to rebook passengers to other flights at this time. The problems don’t appear to be related to any sort of cyber attack, according to a British Airways spokesperson, who spoke with the BBC.

The computer problems are preventing scheduled flights from taking off, and incoming aircraft are unable to park at their gates, leaving passengers stranded on the tarmac. The BBC reports that the outage has resulted in delays at other airports in Rome, Prague, Milan, Stockholm, and Malaga.