British Airways was forced to cancel around 100 flights to and from London 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 have been delayed, according to the BBC.
Sky News reported all check-ins at BA's terminal five at Heathrow had been closed.
BA declined to confirm how many services had been cancelled.
"We are working as quickly as possible to resolve a systems issue which has resulted in some short-haul cancellations and delays from London airports," the airline said in a statement.
"A number of flights continue to operate," it added, apologising to passengers impacted by the disruption and noting back-up manual systems were being used to keep many services in place.
BA said it was offering 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 shared photos showed 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, stated on Twitter.
"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.
"We are working to support the BA team and passengers impacted.
"We understand this is incredibly frustrating for passengers and encourage them to check with British Airways for the latest information."
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.