Britain's air traffic control agency was hit by a technical problem for several hours on Monday, causing widespread disruption to flights across Europe and leading to the cancellation of more than 100 flights into and out of Dublin, Belfast, Cork and Shannon.
The National Air Traffic Service (NATS) said the disruption would continue for some time even though the issue was fixed.
Four Irish athletes returning from the World Championships were caught up in the delays, including Kelly McGrory and Sophie Becker who finished eighth in the 4x400m final on Sunday night. John Fitzsimons and Mark English are also delayed.
The athletes were kept on the runway for three-and-a-half hours before being told their flight was cancelled. They will spend another night in Budapest before flying from Prague on Tuesday. Staff members of Athletics Ireland, fans and members of the media have also been affected by the delays.
The air traffic control agency earlier had to restrict the flow of aircraft when its automatic processing of flight plans malfunctioned, requiring them to be handled manually and causing flight delays and cancellations.
"It was fixed earlier on this afternoon. However, it will take some time for flights to return to normal, and we will continue to work with the airlines and the airports to recover the situation," NATS operations director Juliet Kennedy said in a video posted on its website.
"Our absolute priority is safety and we will be investigating very thoroughly what happened today."
British transport minister Mark Harper said he was working with NATS to help it manage affected flights and support passengers.
Irish air traffic control provider AirNav Ireland earlier 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 severely disrupted and it had made "significant changes" to its schedule, while other airlines, including Ryanair, 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 that warned of delays and cancellations, while Dublin Airport said the problem affected some flights into and out of the capital.
Many passengers earlier 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.
