Britain's National Air Traffic Service (NATS) was hit by a technical problem for several hours on Monday, causing widespread disruption to flights using UK airspace that airlines and airports said would continue for some time despite the issue being fixed.
NATS had earlier had to restrict the flow of aircraft 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."
British transport minister Mark Harper said he was working with NATS to help them manage affected flights and support passengers.
Earlier Irish air traffic control provider AirNav Ireland said the issue, which 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".
A spokesperson for London Heathrow, the busiest hub in western Europe, said schedules would remain significantly disrupted for the rest of the day.
"We ask passengers to only travel to the airport if their flight is confirmed as still operating. Teams across Heathrow are working as hard as they can to minimise the knock-on impacts and assist those whose journeys have been affected," the spokesperson said.
British Airways said 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, London Stansted and London Gatwick were among the many UK airports who warned of delays and cancellations, while Dublin Airport said the issue had resulted 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.