Flights using London airspace are being delayed
Northern Ireland travellers have been hit by delays after computer problems at an air traffic control centre.
A glitch at the control centre at Swanwick, Hampshire, which covers the South East of England has been fixed.
Although services are said to be returning to normal, some delays are still occurring.
It is not yet known if there will be a knock-on effect for travellers on Friday. Live flight information is available from airport websites.
Belfast International Airport and the George Best Belfast City Airport warned of delays for all flights on Thursday using London airspace and some cancellations.
The City of Derry Airport reported only slight delays to flights.
A spokeswoman for Dublin Airport said a number of London flights were affected by the incident.
Laurence Elmore was among those caught up in the delays on a Jet2 flight to Murcia at Belfast International Airport.
He said that they had boarded at 1530 BST, but the airline was confident they would get them to their destination on Thursday evening.
"They have explained there's a problem with the Swanwick radar and they are doing all in their power that they can," he said.
"There is no discomfort or distress, at this point everyone is in good enough humour."
Passengers at Gatwick airport reported cancellations on flights run by budget airline easyJet.
Eamon McGee, a 36-year-old construction consultant from Londonderry, Northern Ireland, said passengers were "queuing around like a big snake".
"There's a queue of about 100 people waiting to speak to someone," he said.
"They've closed the check-in desk, that's not a good sign. There's no-one to ask what's happening. There's a long queue for the easyJet desk. It's not mayhem, but there are a lot of people here.
"There hasn't been any information, I've only heard things from what's on television in the restaurant."
Mr McGee was on his way to Ibiza for a stag weekend when the flights were grounded.
"I'm still hopeful about getting there," he said. "I don't know if we'll make it, there might have to be a change of plan."