Hundreds of flights into and out of the UK have been cancelled following a technical problem affecting the air traffic control provider Nats.
At least 625 flights in or out of the country have been cancelled, according to tracking website FlightRadar24. EasyJet has made 200 cancellations and British Airways has made more than 100.
Nats said on Tuesday evening that its flight systems were starting to recover.
With few aircraft able to take off, it is understood landings are being restricted, leading to some flights being prevented from departing from overseas airports.
At Heathrow, one of the busiest airports in Europe, departures were initially paused around lunchtime. Traffic then resumed, before facing further disruption in the afternoon.
It is understood that the airport ran out of space for incoming flights during the afternoon because planes had not been departing.
Thirteen Heathrow-bound flights were diverted to other airports in the UK and Europe.
Passengers have been advised to contact their airlines to check the status of their flights.
British Airways said in a statement that it was anticipating knock-on effects.
"We were forced to cancel or divert more than 100 flights on Tuesday afternoon," a spokesperson said.
"We're doing everything we can to keep our customers updated and make arrangements for them to continue their journeys.
"We ask customers whose flights have been cancelled not to travel to the airport and to bear with us if they're trying to get in touch, as our call centres are incredibly busy at the moment."
Airports across the UK said they were experiencing cancellations and delays and advised people to check their flight details before travelling.
According to FlightRadar, the worst affected airports are Heathrow, Gatwick, Manchester and Birmingham. The site said approximately 4% of UK flights have been cancelled or delayed as a result of the issue.
Ryanair said affected passengers would be notified.
Knock-on effects as backlog builds
Even if Nats resolves the issue with its flight processing system, the disruption is likely to persist due to backlogs.
Although the technical issue affected departures, arrivals were also hit as space on the ground became increasingly restricted.
As a result there is now a backlog of flights because some planes across Europe are in the wrong place.
Nats said on Tuesday afternoon that engineers were on site, and it was "working urgently to resume normal operations as quickly as possible".
The organisation apologised for the delays caused and advised passengers to check the status of their flight with their airline. Nats said it would provide updates on www.nats.aero and X.
Transport Secretary Heidi Alexander said that "it will take time to fully resume flights, so continue to check with airlines".
"I am seeking assurances that lessons will be learned and systems that support aviation are up to the job," she wrote on X.
According to Nats, the flight processing system processes flight plans and other operational information used by air traffic controllers to manage air traffic.
The system continually updates flight information as aircraft progress through UK airspace, helping controllers maintain a safe and orderly flow of traffic.
Phil White from Bexhill told BBC Your Voice that his flight to Lanzarote boarded on time but has been sitting on the tarmac at Gatwick for hours.
"We've been told the radar is down and we can't take off," Phil said.
"It's comfortable enough but the pilots been on every 20 minutes saying they are waiting for information from air traffic control and when he finds anything out he will let us know."
Chloe Davis, 18, from Cambridgeshire is sitting on a Virgin plane at Heathrow waiting to fly to Orlando.
She told BBC Your Voice that she had been due to depart at 13:40 BST.
"I'm not concerned, I would just like to know what's going on," she said.
"There [are] planes parked up on the taxi way because there's no gates for them to go to."
Chloe is an aspiring pilot and got to talk to the pilot in the cockpit while the plane waited on the ground.
