British Airways said it was relying on backup and manual systems. It canceled 117 flights due to operate to or from Heathrow and 10 Gatwick flights, stranding about 20,000 passengers. More than 300 other flights were delayed.

"A number of flights continue to operate, but we are advising customers to check ba.com for the latest flight information before coming to the airport," the airline said in a statement.

BA said short-haul passengers due to travel Wednesday could re-book for another day.

At Heathrow, Europe's busiest airport, passengers waited in snaking lines for information.

"There were massive queues," said Darren Rowe, whose flight to Hamburg, Germany, was canceled. "It was queue here, queue there. Nobody was saying anything. The lack of information was just pathetic."

The incident is the latest bout of tech trouble to hit the airline. In May 2017, tens of thousands of passengers were stranded when a global computer failure grounded hundreds of British Airways flights over three days. BA passengers were also hit with severe delays in July and September 2016 because of problems with the airline's online check-in systems.

The airline is also set to be fined 183 million pounds ($222 million) by Britain's data regulator over a cyberattack on the airline's security systems last year in which the personal data of up to 500,000 customers was stolen.