London (CNN)British Airways has canceled all flights from London’s two biggest airports for several hours after experiencing “a major IT system failure that is causing very severe disruption to our flight operations worldwide,” the airline said Saturday.

“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.

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 may have caused the problem.

‘Stuck in plane for 1 hour’

Travelers on this spring bank holiday weekend in Britain were already reporting disruptions, including baggage and flight delays, when the airline reported serious computer problems earlier in the day.

Heathrow officials said they were working with British Airways, which is based in the airport’s Terminal 5, “following an issue with their IT system this morning that has caused some delays for passengers.”

“Stuck at Heathrow Terminal 5. No updates or staff around,” travel writer and blogger @WildWayRound tweeted. “Have been stuck in a plane for 1 hour, and now at the airport for 2.”

Staff couldn’t find her baggage, including the gear she needs for a marathon Sunday, she said.

“Long line of planes on the tarmac,” tweeted Don Bowman, of North Carolina, adding that he’d only seen one plane leave Terminal 5 in the previous hour.

Filmmaker Philip Bloom tweeted that he was stuck on the tarmac at Heathrow after his delayed flight from Belfast, Northern Ireland, finally landed because there was no space for the aircraft to park.

CNN’s Simon Cullen contributed to this report.