Heathrow was among several European airports hit by delays on Saturday after a cyber-attack affecting an electronic check-in and baggage system.
The airport said a number of flights were delayed as a technical issue impacted software provided to several airlines.
Brussels Airport said a cyber-attack on Friday night meant passengers were being checked in and boarded manually, and Berlin's Brandenburg Airport reported longer waiting times due to the problem.
RTX, which owns software provider Collins Aerospace, said it was aware of a cyber-related disruption to its system in select airports and that it was working to resolve the issue as quickly as possible.
The company added: The impact is limited to electronic customer check-in and baggage drop and can be mitigated with manual check-in operations. It said its Muse software - which allows different airlines to use the same check-in desks and boarding gates at an airport, rather than requiring their own - had been affected.
The BBC understands that British Airways is operating as normal using a back-up system, but that most other airlines operating from Heathrow have been affected.
Shortly before midnight on Saturday, Heathrow said it was continuing to resolve and recover from the outage. It apologised to those who had faced delays but stressed that the vast majority of flights have continued to operate.
A National Cyber Security Centre spokesperson said it was working with Collins Aerospace, affected UK airports, the Department for Transport and law enforcement to fully understand the impact of the incident.
The European Commission, which plays a role in managing airspace across Europe, said it was closely monitoring the cyber-attack. A spokesperson added that there was no indication it had been widespread or severe and that it was working with airlines and airports to restore operations and support passengers.
Hundreds of flights have been delayed at the airports throughout Saturday, according to flight tracker FlightAware. Dublin Airport reported minor impacts, implementing manual check-in processes.
As passengers faced long queues and frustration, some expressed their hardships, including missing connections and extended waits, amidst chaotic scenes at various terminals.
