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 on Sunday that efforts to resolve the issue were ongoing, apologizing to those who had faced delays but stressing that the vast majority of flights have continued to operate, urging passengers to check their flight status before travelling to the airport and arrive in good time.
The BBC understands that British Airways has continued to operate as normal at the airport using a back-up system, but that most other airlines that service it had been affected by the outage.
There were hours-long queues on Saturday and some 47% of Heathrow's departing flights were delayed, according to flight tracker FlightAware. Additional staff were at hand in check-in areas to help minimise disruption.
Passengers have reported significant wait times, with some missing connecting flights due to the delays.
On Sunday, Brussels Airport announced that manual check-in would continue and that extra staff had been brought in to manage the situation.
Europe's aviation safety organisation has advised airlines to reduce their flight schedules in light of the ongoing disruptions.
The National Cyber Security Centre is currently assessing the situation with Collins Aerospace and affected airports to understand the impact of the incident.
Travelers are encouraged to stay updated directly with their airlines regarding flight status.
