LONDON (AP) — Britain's air traffic controller says a technical problem is causing some flight restrictions.

The main air navigation service provider in the United Kingdom did not elaborate on what was causing the issue and it was not immediately clear if the matter was weather related.

The controller, NATS, says the incident occurred at its Swanwick Control Center, southwest of London.

The incident comes as passengers at Heathrow and Gatwick airports experienced delays and cancelations. Record heat on Thursday made way for thunderstorms on Friday.