Passengers travelling from two of Northern Ireland's airports are facing delays and cancellations because of an air traffic control centre fault.
Flights are being delayed at airports across the UK and in the Irish Republic.
The National Air Traffic Control Service (Nats) said a technical problem at its Swanwick centre, in Hampshire, had led to the delays.
It was caused as a result of "difficulty switching from night time to daytime operation".
Neil Breen, who is travelling with his fiancée to Portugal to make wedding arrangements, was stuck on board a grounded plane at Belfast International Airport for an hour and a half before it took off.
"We were waiting on the flight, aware there's been quite a few delays, we were basically put on the plane and the information was delivered that we'd be sitting here on the runway for at least another hour," he said.
"I think everybody was just getting a bit antsy, they just want to get on with their holiday."
Passengers due to fly on Saturday should check with their airline before travelling to the airport.
Ryanair has warned there will be "significant flight delays and possible cancellations".
British Airways said the technical problems had already led to flight cancellations and warned that they "will cause delays to some flights".
Travel expert Simon Calder said the delays were likely to have a knock on effect during the day.
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