The airline advised passengers against coming to the airports as it would not be able to help them.
British Airways refuted media claims that the outage had been caused by a cyber attack, saying that there was no evidence to support that.
Earlier, the company posted a tweet recognizing the problem.
We apologise for the current IT systems outage. We are working to resolve the problem as quickly as possible.— British Airways (@British_Airways) May 27, 2017
The official Twitter account of Heathrow Airport, the busiest airport in Europe, reported that there was an issue.
We're working with @British_Airways following an issue with their IT system this morning that has caused some delays for passengers. 1/2— Heathrow Airport (@HeathrowAirport) May 27, 2017
Passengers who had already arrived at the airports were less than pleased with the chaos that had erupted there.
Social media posts from Heathrow show long queues to the British Airways check-in desks.
Unable to load the luggage on the planes, the company left the bags temporarily near its counters.
British Airways is the latest airline to be facing grave IT issues. Last month, Germany's Lufthansa and Air France suffered a global system outage which prevented them from boarding passengers at the time.