"U.K. airspace has not been closed, but airspace capacity has been restricted in order to manage the situation. ," said NATS, a U.K.-based global air traffic management company.
"We apologise for any current delays. Every possible action is being taken to resolve the situation," NATS said.
Heathrow Airport, Europe's busiest, advised travellers via to check their flight status before heading to the airport.
Gatwick Airport said flights are now departing, but added there are delays.
Some media reports suggested that London airspace had been closed entirely.
NATS control centre said a technical problem was reported at Swanick air traffic control centre.
The problem affects five commercial airports, including Heathrow, Gatwick, Luton, Stansted and London City Airports.