Heathrow is among several European airports hit by a cyber-attack affecting an electronic check-in and baggage system.
The airport warned of possible delays due to a technical issue affecting software provided by Collins Aerospace to several airlines.
Brussels Airport reported that a cyber-attack on Friday night meant passengers were being checked in and boarded manually, while Berlin's Brandenburg Airport also experienced longer waiting times.
RTX, which owns Collins Aerospace, stated it was aware of a cyber-related disruption and is working to resolve the issue as quickly as possible.
The company's statement indicated that the disruption was limited to electronic check-in and baggage drop processes, which could be mitigated with manual operations.
The Muse software impacted allows airlines to share check-in desks and boarding gates at airports.
British Airways is operating normally with a backup system, but most other airlines at Heathrow are affected, with hundreds of flights experiencing delays throughout Saturday, according to FlightAware.
Passengers described long queues, with one traveler reporting over two hours waiting to check in. Linda Spencer recounted that staff were manually tagging luggage and checking in passengers via phone.
Another traveler, Monazza Aslam, expressed frustration after being stuck on the tarmac for over an hour and missing a connecting flight.
Heathrow has placed additional staff in check-in areas to manage the disruptions and is advising passengers to check their flight status with airlines.
Transport Secretary Heidi Alexander is monitoring the situation closely.
The incident also prompted cancellations and delays at Brussels Airport, affecting the orders of half the flight schedules. Similar issues are reported at Dublin Airport, where one terminal was evacuated as a precautionary measure.
Travel experts warn that disruptions at these key hubs could lead to cascading effects across European air travel.
Although there are unverified claims linking this attack to Kremlin-sponsored hackers, many recent cyber incidents have been traced to criminal gangs focused on extorting money, often via ransomware.
As investigations continue, Collins Aerospace has yet to provide a detailed statement regarding the nature of the cyber attack.