Flights have been cancelled, delayed or diverted because of a lack of staff in air traffic control at Gatwick airport.
Dozens of arrivals and departures were affected on Thursday evening, with passengers advised to contact their airline.
National Air Traffic Services (Nats) said: "Air traffic control restrictions have been put in place this afternoon due to a short notice staff absence affecting our air traffic control team at Gatwick Airport.
"We are working closely with the airport to ensure we can handle flights with as little disruption as possible and we apologise very sincerely to people who have been inconvenienced [as a result of unavoidable diversions].
"We are working closely with Gatwick Airport Ltd to build resilience in the airport's control tower to ensure disruption is kept to a minimum.
"New air traffic controllers have been recruited since last summer, increasing our presence by 17%, and others are due to start after completing their training, in line with the agreed plan when Nats took over the contract last October.
"London Gatwick's senior management understands that we are working hard to keep the operation moving. Airlines operating at London Gatwick were aware of the situation when Nats was appointed but that does not dilute the apology we offer sincerely to them and their passengers who have been inconvenienced by recent disruption."
The Sussex airport apologised, adding in a statement: "Nats are a world-class provider of air traffic services and London Gatwick's senior management recognises how hard the airport's air traffic controllers are working to keep the operation moving.
"We are working closely with Nats to build resilience in the airport's control tower to ensure disruption is kept to a minimum."
It comes after the Nats control system for the entire UK was hit by a technical glitch on Bank Holiday Monday, August 28, causing widespread disruption.
More than a quarter of flights to and from UK airports were cancelled that day, affecting around 250,000 people.
Cancellations continued for two more days as planes and crews were out of position.