Flights using United Kingdom airspace have been delayed or cancelled for several hours due to what the country's National Air Traffic Service says was a technical issue that it has now identified and remedied.
NATS had earlier had to restrict the flow of aircraft and manually input flight plans after the issue affected its system's ability to automatically process flight plans, with airlines and airports warning of delays and cancellations.
"We have identified and remedied the technical issue affecting our flight planning system this morning. We are now working closely with airlines and airports to manage the flights affected as efficiently as possible," NATs said in a statement.
"Our engineers will be carefully monitoring the system's performance as we return to normal operations."
Earlier Irish air traffic control provider AirNav Ireland said the issue, which struck during a public holiday in parts of the UK, was resulting in "significant delays for flights across Europe that are travelling to, from or through UK airspace".
A representative for London Heathrow, the busiest hub in western Europe, said the airport was working with NATS and other airport partners to minimise the impact on passengers while Gatwick, south of London, said it was dealing with multiple delays and cancellations.
Earlier, Scottish airline Loganair said there had been a network-wide failure of UK air traffic control computer systems.
British Airways said it its flights were being severely disrupted and it had made "significant changes" to its schedule while other airlines including Ryanair also said some flights to and from the UK would be delayed or cancelled.
Manchester Airport and London Stansted were among the many UK airports who warned of potential disruption to flights while Dublin Airport said the issues were resulting in delays and cancellations to some flights into and out of the Irish capital.
Many passengers took to social media to say they were stuck on planes on the tarmac waiting to take off, or being held in airport buildings, in Spain, Portugal, Greece, Israel and elsewhere on what is a traditionally busy travel day as the school holidays draw to a close.
One Reuters witness who was held on the tarmac at Budapest for two hours before being taken off the plane said their pilot told passengers that they could face an 8-12 hour delay.