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 said a cyber-attack on Friday night meant passengers were being checked in and boarded manually, while Berlin’s Brandenburg Airport also reported longer waiting times due to the problem.
RTX, which owns 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.”
Heathrow said additional staff were on hand in check-in areas to help minimise disruption.
It added: “We advise passengers to check their flight status with their airline before travelling to the airport and arrive no earlier than three hours before a long haul flight or two hours for a domestic flight.”
Brussels said there would be a “large impact on the flight schedule”, including cancellations and delays.
Europe’s combined aviation safety organisation, Eurocontrol, said airline operators had been asked to cancel half their flight schedules to and from the airport between 04:00 and 12:00 GMT due to the disruption.
It said “similar issues” were affecting Heathrow and Berlin, and that “measures may be required”.
There have been nearly 100 delays in and out of Heathrow as of 10:00 BST on Saturday, according to tracker FlightAware, while there have been nearly 70 delays in Brussels and 15 in Berlin.