Flight disruption sparked by a suspected cyber attack is continuing at London Heathrow Airport, with a number of flights delayed on Monday.
A “technical issue” hit boarding systems used by airlines on Friday night, with affected transport hubs across the continent registering passengers manually using pen and paper.
RTX, which owns software provider Collins Aerospace, said it was "aware of a cyber-related disruption" to its Muse software and that it hoped to resolve the issue as soon as possible.
The technology allows multiple providers to use the same check-in desks and departure gates at an airport, rather than requiring their own.
But Collins Aerospace, which boasts NATO among its clients, has yet to disclose what went wrong or how long the outage is expected to last.
Brussels Airport said it had "no indication yet" when the system would be operational again and requested airlines to suspend half its departing flights for Monday.
Meanwhile, Heathrow in Hillingdon, west London revealed at the weekend that efforts to resolve the issue were “ongoing” after a dozen flights were cancelled and more than 200 were delayed, according to flight tracking data.
A number of flights from Heathrow were showing as delayed on Monday morning, with planes heading to Stuttgart, Dublin and Zurich among those affected.
Passengers due to fly from the airport’s Terminal 4 said they were met with queues, delays and confusion as to whether they would be able to make their planned trips.
Heathrow apologised to those who had faced delays but stressed "the vast majority of flights have continued to operate".
It is understood British Airways at Terminal 5 remains unaffected and has been operating as normal.
A spokesman for Dublin Airport, which has also been suffering problems from the IT meltdown, told the BBC: "Passengers are advised to contact their airline directly for updates on their flight."
And a statement on the Berlin Brandenburg Airport website said there were longer waiting times and advised passengers to use self-service check-in prior to arrival.
The National Cyber Security Centre is currently working with Collins Aerospace and affected UK airports, alongside the Department for Transport, “to fully understand the impact of an incident”.
The European Commission, which plays a key role in managing airspace across the continent, said it was "closely monitoring” the situation, but that there was no indication it had been "widespread or severe".
Questions are now being asked as to what caused the global IT meltdown, with politicians and aviation experts appearing to point the finger at the Kremlin.
The Liberal Democrats’ foreign affairs spokesman Callum Miller told Sky News: “We think it's right that the government should come clean and let us know what its assessment is — of whether or not this was indeed an attack by Russia."
It comes a year after a faulty software update from cybersecurity company Crowdstrike halted flights across the United States, costing the global economy up to $10 billion (£7 billion).
