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 - had been affected.
British Airways is reportedly unaffected, but most other airlines at Heathrow have been impacted. Heathrow stated it was working to resume normal operations and apologized for the delays.
A spokesperson from the National Cyber Security Centre said they were collaborating with affected airports and law enforcement to assess the situation. Meanwhile, the European Commission is monitoring the attack, which reportedly has not been severe.
Flight tracking services reported hundreds of delays, with passengers sharing their frustrating experiences at various airports. Many are now concerned about their travel plans, particularly those affected by missed connections.
While investigations are ongoing, speculation surrounds the motives and origin of the attack, with some experts suggesting possible links to ransomware gangs.