A cyberattack on a provider of check-in and boarding systems has disrupted operations at several major European airports, including London’s Heathrow — the continent’s busiest — causing flight delays and cancellations on Saturday.

Collins Aerospace, which provides systems for several airlines at airports globally, is experiencing a technical issue that may cause delays for departing passengers, Heathrow Airport said, having warned of delays.

Brussels Airport and Berlin Airport were also affected by the attack, they said in separate statements.

RTX, Collins Aerospace’s parent company, said it had become aware of a “cyber-related disruption” to its software at selected airports, without naming them.

Electronic check-in affected

“The impact is limited to electronic customer check-in and baggage drop and can be mitigated with manual check-in operations,” RTX said in an e-mailed statement, adding that it was working to fix the issue as quickly as possible.

The attack has rendered automated systems inoperable, allowing only manual check-in and boarding procedures, Brussels Airport said on its website, adding the incident had occurred on Friday night.

“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.

Frankfurt Airport not impacted

“Due to a technical issue at a system provider … 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.

EasyJet, among Europe’s biggest airlines, said it was currently operating as normal and did not expect the issue to impact its flights for the rest of the day. Other major airlines Ryanair and British Airways owner IAG did not immediately respond to Reuters’ requests for comment.

There were no indications of threats to Polish airports, Deputy Prime Minister and Digital Affairs Minister Krzysztof Gawkowski said.