Collins Aerospace, which provides check-in and boarding systems for several airlines across multiple airports globally, is experiencing a technical issue that may cause delays for departing passengers, London’s Heathrow Airport said on Saturday, having warned of delays.

RTX, Collins Aerospace’s parent, was not immediately available for comment outside of US business hours.

The attack has rendered automated systems inoperable, allowing only manual check-in and boarding procedures, Brussels Airport said on its website.

“This has a large impact on the flight schedule and will, unfortunately, cause delays and cancellations of flights…The service provider is actively working on the issue and trying to resolve the problem as quickly as possible.”

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, Germany’s largest, was not affected, a spokesperson said. An official from the operations control centre at Zurich Airport also said it had not been impacted.