An investigation has been launched into an air traffic control system failure that resulted in the UK's airspace being restricted on Friday, causing travel chaos for thousands of passengers.
The focus of the probe will be the computer systems that manages the flight paths of planes, based in the headquarters of the air traffic control service NATS in Swanwick, Hampshire.
The failure resulted in more than 300 flights being cancelled or delayed yesterday.
"Following a technical fault with the flight data system used by air traffic controllers at Swanwick, Nats can confirm that the system has been restored to full operational capability and a thorough investigation is continuing, to identify the root cause.
"Although operational restrictions applied during the failure have been lifted, it will take time for flight operations across the UK to fully recover," Nats said in a statement.
It is the second major failure of the Nats system in just over a year, after thousands of flights were disrupted following problems with Nats' systems in December 2013.
This morning 40 flights due to take off from Heathrow before 9.30am, and a handful at Gatwick, were cancelled, as the fallout of the systems failure continues.
Airports throughout the country were affected by the failure, including Manchester, Birmingham, Stansted and Luton, and as far north as Edinburgh and Aberdeen. More than 10,000 passengers experienced disruption.
Transport secretary Patrick McLoughlin called the failure "unacceptable".
McLoughlin said: "Any disruption to our aviation system is a matter of the utmost concern, especially at this time of year in the run up to the holiday season.
"Disruption on this scale is simply unacceptable and I have asked Nats for a full explanation of this evening's incident. I also want to know what steps will be taken to prevent this happening again."
Experts said that Nats' system is outdated, and small malfunctions could have huge knock-on effects.
Air traffic control safety expert Philip Butterworth-Hayes told Sky News: "Swanwick… has an incredibly complicated, customised software system, where you have to upgrade things all the time. A small upgrade can cause all sorts of problems.
"The issue we have in the UK, especially in the southeastern UK, is there's very little spare capacity, because the system is running at such high rates, it only needs a small little glitch to create a large problem."