The attack has rendered automated systems inoperable, allowing only manual check-in and boarding procedures, according to Brussels Airport, Reuters reported.
“This has a large impact on the flight schedule and will unfortunately cause delays and cancelations of flights,” the operator said in a statement published on its website.
“The service provider is actively working on the issue and trying to resolve the problem as quickly as possible.”
Heathrow also warned of delays caused by “a technical issue” at a third-party supplier.
Passengers with a flight scheduled for Saturday were advised by the affected airports to confirm their travel with airlines before heading to the airport.
“Due to a technical issue at a system provider operating across Europe, there are longer waiting times at check-in. We are working on a quick solution,” Berlin airport said in a banner on its website.
Frankfurt Airport is not affected by the attack, a spokesperson said.