Italy’s Air traffic control services are down after Britain’s National Air Traffic Service (NATS) had to restrict the flow of aircraft and manually input flight plans on Monday due to a technical issue, it said, with airlines and airports warning of delays and cancellations across Europe.

It is not yet clear whether the problem is due to a malfunction of the air traffic control system in Italy as well or to a hacker attack.

The consequence of the problem is that flights to Italian airports and from western Italy to other parts of Europe are being significantly delayed.

The problem started at around 1 p.m. with the collapse of the air traffic control system in Britain. This has resulted in long delays for flights from British airports to other countries – including Greece – and equally long delays for flights from European airports outside Britain, such as Eleftherios Venizelos, to airports in Great Britain.

This is illustrative of what is happening with an Aegean flight that was due to depart at 13:30 from Eleftherios Venizelos to London. At 15:30, after two hours on board, the dozens of passengers were instructed to disembark back at the airport, where they are still waiting without knowing the time of the flight to the British capital. According to reports, London is instructing all airlines to cancel flights because it will be very difficult to normalise the situation even after several hours have passed.

“We are currently experiencing a technical issue and have applied traffic flow restrictions to maintain safety. Engineers are working to find and fix the fault,” NATS said in a statement, adding that UK airspace was not closed.

NATS said the issue was affecting its ability to automatically process flight plans.

“Until our engineers have resolved this, flight plans are being input manually which means we cannot process them at the same volume … Our technical experts are looking at all possible solutions to rectify this as quickly as possible.”

Irish air traffic control provider AirNav Ireland said the issue, which has struck during a public holiday in parts of Britain, was resulting in “significant delays for flights across Europe that are travelling to, from or through UK airspace”.