Thousands of air passengers are facing delays after an air traffic control computer failure caused flights to be suspended across the UK.
Air safety was said to be unaffected by the computer failure
National Air Traffic Services said flights were grounded so that controllers could prioritise on planes in the air, but safety was unaffected.
The air traffic control centre at West Drayton is now fully operational again and flights are resuming.
Many airports are advising people to check in as normal.
Nats' Flight Data Processing System failed at around 0600BST for an hour.
Speaking to BBC News 24 Chief Executive Richard Everitt said the failure followed overnight testing of an upgrade to its Flight Data Processing System in West Drayton.
"This is a significant upgrade that we will be doing later in the year, we have to test that very thoroughly because safety is paramount.
"We will now investigate why there was this problem - clearly it was not an anticipated problem - a lot of work will be done today to understand why we had problems with this testing," he said.
"What we did here was to ensure safety levels were maintained, we did that by taking the traffic right down, handling the arrivals while we sorted this problem out. "
Mr Everitt said Nats was planning on spending over £1bn on upgrading its system over the next eight years.
He apologised to passengers and said Nats was working with airlines to minimise disruption.
Speaking on BBC Radio 4's Today programme, Transport Secretary Alistair Darling insisted Britain's system compared favourably with other countries.
"We are putting the money in, it is making a difference.
"If you look at the delays caused by air traffic control failure they have come down dramatically over the last few years," he added.
BBC transport correspondent Tom Symonds says the affected computer systems were not those that handle the separation of aircraft in the air.
He says the fault was in what is known as the host control system, a two-year-old computer system at West Drayton.
The system handles flight strips which are the basic details of flights coming in and out of the UK.
Spokesman for London's Heathrow Airport, Mark Pearson, said: "There is serious disruption at Heathrow as a result of the air traffic control situation.
"There is an average of two hour delays on departures and restrictions on arrivals."
He advised passengers to check with their airlines before leaving home.
Passengers Monica and Gerald Godfrey - waiting for a British Airways flight from Heathrow to Toronto - told BBC News Online their flight had been delayed by the problems.
The couple said they would have a cup of tea and relax after checking in.
"It depends on how long the delay is, we might get a little uptight," Mrs Godfrey said.
Mr Godfrey added: "But at the end of the day there's nothing we can do about it.
"As long as there is an aircraft and someone waiting for us at the other end we'll be fine."
Have you been affected by the delays? Send us your experiences using the form below.
"Computer failure" blamed again. I hope one day we'll hear whether this was a genuine computer failure, or a human error. If the former, why were hot backups not immediately available? If the latter, will heads roll? Simply blaming "computer failure" may have been acceptable in the '70s, but this is 2004; no one point of failure - computer or otherwise - should be able to cause this much chaos.
I was affected and decided to cancel my business trip, however I'd sooner have a few hours delay and know my safety once in the air was in good hands and being controlled by the latest computer technology. An upgrade is essential to make sure we all remain safe.
Ricky, Milton Keynes, Bucks
There seemed to be a lot of conflicting and perhaps premature information being handed out at Manchester this morning.
I arrived between 6 and 7 and was told that unless travel was essential, we should go home and rebook. This view was echoed by several airline staff from the same airline. As I had a short connection at Heathrow before my flight to North Africa, I took the advice. I believe the 'don't travel' advice was repeated on TV. I returned home, and then found that connecting flights had made it, and the flight to N.Africa left only moderately late - meaning I have a 2 day wait for the next flight.
Disappointing is not the word
Steve Roden, Liverpool
0830 BA Flight to Belfast City delayed and then cancelled for technical reasons. I gave up and went back home to conduct my meetings by telephone.
Mike Hardcastle, Manchester UK
This is so typical of many services within the UK. My sister was due to fly from Dublin to Stanstead and then catch a connecting flight to Dinard France. She was delayed for 3 hours in Dublin has now missed her connecting flight and is now stuck in London. She is unable to catch a later flight as there is only one flight a day to Dinard. It cost 200 euros for the ticket to come and she has to pay for a return flight to Dublin. Apologies do little for this family and now I will have no family here to help me celebrate my daughters first birthday....
Anne Marie O'Rourke , Rennes France
My inbound flight from JFK landed at early at 0515 - What a stroke of luck!
Brian O'Connor, Blackburn Lancs
I was on my way to my flight to Bari in Italy this morning. Only to find from the radio, that my flight from Heathrow, which was scheduled for 6.30 was running two hours late. I had no choice but to abandon my meeting. This is a major inconvenience.
Mahmood Snipe, Braintree
What were they doing testing an upgrade on a live computer? The IT managers responsible should be sacked for incompetence of the highest order.
Bob Harris, Bristend, Herts
With regard to Bob Harris's comments, I have worked in the IT industry for 15 years and no matter how much testing you carry out, the final and only test that really means anything is when the system goes live. Only when a system is live, can it be fully stressed, if faults appear then they are real and can be identified and put right far quicker than endless testing on guessed failures. I doubt if there was any danger to any aircraft whilst this went live as the engineers involved would be half expecting a failure of some description. Anybody who puts 100% trust in a piece of electronic/software wizardry should really get out more!
D Kinstrey, Horley,UK
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