Passengers faced long delays after more than 200 flights were cancelled at UK airports.
Thousands of passengers have been left stranded on planes and airports across Britain after a fault at air traffic control centre.
226 flights have been cancelled at Heathrow; 5 were cancelled at Gatwick, with a further 86 flights delayed for one hour; and flights at Stanstead were delayed by up to two hours.
There were 20% fewer flights today (7 December), with 2,800 departures, as opposed to the usual 3500 flights on a Saturday.
The problem arose at the National Air Traffic Service's (Nats) Swanick Centre, when the internal phone system, which monitors flights all over England, broke down, preventing the computer switching from night to day mode, meaning fewer flights could be monitored.
Nats operations manager, Juliet Kennedy told the BBC: "At night we manage quite big sectors of airspace because the traffic demand is low. During the day, as demand increases, we need to split the airspace into smaller sectors - last night we were unable to open new positions. It was a problem with the internal telephone system."
Short-haul flights have been worst affected, since cancellations of long-haul flights would have wider implications for international arrivals.
While the airports run a reduced service, passengers have reported delays of up to five hours, causing widespread inconvenience and ruining the weekend getaway for many families. Travellers have been further frustrated due to the lack of information regarding flight updates.
The airlines too, have expressed their discontent. With 100 flights affected and 10 flights cancelled, RyanAir has called on the Civil Aviation Authority to take action.
"While we acknowledge problems can occur, where is the contingency? It's simply not good enough and the CAA needs to act now," said RyanAir in a statement.
With engineers battling to rectify the fault, it was hoped that normal service would resume after 6pm, but with a backlog of flights, there is no clear sign as to when the matter will be resolved. The disruption is expected to continue for several hours yet.
Reassuring passengers that the fault had no impact on safety, Kennedy added: "We want to apologise to people. We are very aware of how much inconvenience this has caused people."
Belfast, Edinburgh, Manchester, Birmingham, Southampton, Luton, London City, Newcastle, Exeter and Bournemouth are among other airports that have reported delays. Passengers have been advised to check for further travel disuptions with the airlines.
Nats said that further disruption to flights on Sunday (8 December) was "almost inevitable."
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