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. It apologised to those who had faced delays but stressed that 'the vast majority of flights have continued to operate', urging passengers to check their flight status 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.
Lucy Spencer told the BBC that she had queued to check in for a Malaysia Airlines flight for more than two hours, and that staff had been checking passengers in over the phone.
Another passenger, Monazza Aslam, said she had to wait since the early hours with her elderly parents, and that the delays meant they had missed their connecting flight.
Brussels Airport said manual check-in would continue on Sunday, drafting in extra staff to help minimise disruption. It said 44 departing flights had been cancelled on Sunday alone, anticipating long queues and further delays.
Meanwhile, Dublin Airport indicated that while the technical issues continued, it was expecting to operate a full schedule on Sunday. Travellers are advised to contact their airline directly for updates.
A spokesperson with the National Cyber Security Centre confirmed they are collaborating with Collins Aerospace, affected UK airports, and law enforcement to understand the incident's impact. The European Commission noted the cyber-attack is being closely monitored.
Transport Secretary Heidi Alexander is receiving regular updates regarding the situation.
This incident illustrates a growing concern over how the aviation sector can be impacted by digital system vulnerabilities.
