The UK's Air Traffic Control operator says it has identified and fixed a technical issue that caused disruption today.
It says the problem was affecting the automatic loading of flight plans, so they had to be input manually.
It's working with airlines and airports to get through the effected flights as efficiently as possible.
However, the issue has already delayed and cancelled some flights in and out of Ireland, including Ireland West Airport Knock.
Junior Transport Minister Jack Chambers says those intending to travel should contact their airline....