British Airways was forced to cancel around 100 flights to and from London airports on Wednesday after it suffered "a systems issue".
Reports suggested at least 18,000 passengers were grounded after the services in and out of Gatwick and Heathrow -- Britain's busiest airports -- were cancelled.
More than 200 other BA flights were delayed, according to the BBC. The airline declined to give a number of flights affected.
BA blamed "a systems issue which has resulted in some short-haul cancellations and delays from London airports".
It later said the issue had been resolved, and apologised to affected passengers.
"Our teams have been working tirelessly to get the vast majority of customers on their way, with most of our flights departing," it said.
"Our flights are returning to normal, however there may be some knock-on operational disruption as a result of the issue earlier."
BA had offered customers booked to travel Wednesday on short-haul flights departing from Heathrow, Gatwick and London City airports the chance to rebook to travel on another day between August 8-13.
Frustrated passengers took to social media to vent their anger and demand answers from BA, while some shared photos showing airport terminals filled with stranded travellers.
"There is no staff at the airport, we're in an info queue that isn't moving," Anya, a BA passenger bound for Zagreb, Croatia, tweeted.
"The only staff around are the duty free staff and they've been more helpful than anyone at BA. Get your act together #britishairways!"
A Heathrow spokesperson said BA was experiencing "a technical issue with its systems" impacting check-in and departures there.
The disruption is the latest incident to impact British Airways in recent years.
A 2017 worldwide system outage affected tens of thousands of its passengers, while a 2018 cyber attack saw hundreds of thousands of customers' personal data stolen.