Air travellers are facing another day of disruption at several European airports including Heathrow, after a cyber-attack knocked out a check-in and baggage system.
There were hundreds of delays on Saturday after the software used by several airlines failed, with affected airports boarding passengers using pen and paper.
Brussels Airport said it had no indication yet when the system would be functional again and had asked airlines to cancel half their departing flights.
RTX, which owns software provider Collins Aerospace, said it was aware of a cyber-related disruption to its system in select airports and that it hoped to resolve the issue as quickly as possible.
It identified its Muse software - which allows different airlines to use the same check-in desks and boarding gates at an airport, rather than requiring their own - as the system that had been affected.
The company has yet to disclose what went wrong or how long it expects the outage to last.
Heathrow said efforts to resolve the issue were ongoing, apologizing for the delays but noting that the vast majority of flights have continued to operate. Passengers are urged to check their flight status before traveling to the airport and to arrive in good time.
The BBC understands that British Airways has continued to operate normally at the airport using a backup system, although most other airlines have been affected by the outage.
Saturdays were chaotic at Heathrow, with hours-long queues reported, and around 47% of flights delayed, according to flight tracker FlightAware. Additional staff were deployed to check-in areas to alleviate disruptions.
Brussels Airport has implemented manual check-in and brought in additional staff to help minimize the impact, predicting long lines and further delays.
The National Cyber Security Centre announced it was working with Collins Aerospace and affected airports to fully understand the incident's impact.
Transport Secretary Heidi Alexander is closely monitoring the situation, and the European Commission stated that it is watching the attack but reports no severe issues at this point.