At least 81 flights were canceled at Heathrow while at least 10 suffered the same fate at Gatwick Airport, according to BBC News.
The airline was forced to revert to manual check-in for some flights, BBC reported.
Delayed passengers were not happy.
“How many more incidents before you get your act together?” one flier tweeted.
“What is going on @British_Airways- things were improving! Now we’re back to shoddy IT, delays, strikes and unhappy crew,” another wrote.
British Airways apologized in a statement Wednesday.
“We are very sorry to our customers for the disruption to their travel plans,” the airline said. “We are working as quickly as possible.”
They said some flights are still operating and advised passengers to visit their website to check the flights’ status.
The airline is allowing customers on commuter flights to rebook for another day.