London (CNN) -- A technical issue at the UK's air traffic control center in Swanwick is causing delays to hundreds of flights Saturday across the United Kingdom, including Heathrow Airport.
The National Air Traffic Services said in a lunchtime statement that it had identified the problem and anticipated it would take until 6:30 p.m. (1:30 p.m. ET) to resolve.
An issue with the center's internal telephone system has resulted "in a significant reduction in capacity in some areas of UK en-route airspace," it said. However, "Safety has not been compromised at any time."
NATS would normally have handled about 2,000 flights by noon on Saturday but was about 20% down as a result of the morning's problems, it said, with about 1,700 handled.
"We now understand what the problem is and our engineers are working hard to rectify the issues as quickly as possible," the statement said.
The NATS center at Swanwick handles flights for much of England and Wales, including the airspace around London which it says is one of the busiest areas in Europe.
Stranded travelers voiced their frustration on Twitter.
"Flight changed and pushed back twice. Delay of just two hours. Let's hope it stays that way," said Carolanner.
A Heathrow spokeswoman said, "Flights from many UK airports, including Heathrow, are subject to delay and cancellation. If you are flying today you should check the status of your flight with your airline."
Airport information boards indicated the problem is affecting departures to a greater extent than arrivals.
Heathrow, to the west of London, is one of the busiest airports in the world, serving 70 million passengers annually according to its website. On average, around 191,000 passengers transited the airport daily in 2012.
Gatwick, Stansted and Luton Airports in southeast England are also suffering disruption, as are other busy UK airports, including Birmingham, Manchester, Glasgow and Edinburgh.
Dublin Airport in Ireland said it was also experiencing some flight delays and cancellations as a result of the problems in the United Kingdom.
Low-cost airline Ryanair said on its website, "Ryanair has been advised of an equipment failure within UK Air Traffic Control which will cause significant flight delays and possible cancellations."
Transatlantic operator Virgin Atlantic said it was "experiencing some delays," while shorthaul airline Easyjet said it was "experiencing severe delays to flights to, from and within the UK."
British Airways said the problem had caused delays to some flights and "led to significant shorthaul cancellations."
NATS said the technical problem involved its internal communications system.
"At night, when it's quiet, sectors of airspace are combined. As it gets busier in the daytime the sectors are split out again and additional control positions are opened to meet the traffic demand.
"Because of the problem with the internal telephone system, it was not possible to open the additional control positions this morning, resulting in a significant reduction in capacity in some areas of UK en-route airspace."