British Airways has canceled all flights from London’s Heathrow and Gatwick airports as a global IT failure causes severe disruption for travelers.

The airline says it has suffered a “major IT systems failure.”

BA says terminals at Heathrow and Gatwick have become extremely congested and it is cancelling all flights from the airports until 6 p.m. It is urging passengers not to go to the airports.

Earlier, passengers at Heathrow reported long lines at check-in counters and flight delays.

BA has not said what is causing the computer problem, but says it is working to resolve it as quickly as possible.