Passengers were forced to join long queues at Heathrow Terminal 5 as a technical problem at an air traffic control centre continued to cause chaos at airports across the UK and Ireland.
The £623 million state-of-the-art air traffic control centre at Swanwick in Hampshire has previously experienced technical difficulties since air traffic control company Nats moved in.
In the first few months of its opening in 2002, Swanwick experienced a number of computer failures prompting MPs to question staffing levels at the company.
There were also problems in September 2008 when a computer fault led to flight delays, while in July this year flights in southern England were delayed due to another problem at the centre.
National Air Traffic Services have identified the technical problem which has resulted in travel chaos in airports across the UK.
The company said a problem with the internal telephone system meant additional control positions were not able to be opened to cope with the demand in traffic.
"We now understand what the problem is and our engineers are working hard to rectify the issues as quickly as possible. Everyone in our operation will continue to work with our customers to manage the traffic and clear the backlog," the statement read.
The company insisted safety had "not been compromised at any time" and the disruption had only caused a reduction of around 20 per cent.
"We apologise sincerely for any inconvenience being caused to passengers," Nats added.
Take That star Howard Donald has been caught up in the flight disruption caused by a technical problem at an air traffic control centre.
Delays caused by air traffic control problems could last for the entire day, an aviation expert has warned.
Independent aviation analyst Chris Yates said: "It's going to be a day of frustrations and the knock-on effects are going to last for the whole day because of the backlog of planes. It will be a tough day for everybody.
"There are contingency plans in place whenever this happens. Many of the long-haul flights, coming from China, India, the US and so on, passengers sitting on those planes may find themselves diverted to continental airports.
"But it's going to be a long wait for them. When the system kicks back in and starts working, there will be a backlog of flights.
"For those waiting to fly out, it's going to be a case of sitting around the airport terminal until things get back to normal."
The technical problem affecting the Swanwick air traffic control centre is unlikely to be fixed before 2pm.
Thousands of people have been affected by delays at major airports including Stansted, Heathrow and Gatwick.
The Association of British Travel Agents (Abta) said: "Technicians are working to try to resolve the issue but have advised that this is unlikely to be before 2pm today.
"Delays and flight cancellations at some UK airports are being experienced as a result. Customers due to travel today should liaise with their airline to establish if there are any changes to their flight arrangements."
The association said customers should be prepared for potential delays and advised them to carry essential items such as medication in their hand luggage.
Daisy McAndrew, who is stranded on a plane at Gatwick, said passengers had been "given no indication" of how long it would take to solve the air traffic control problem.