Flights are now leaving Heathrow after the delays, but it will take some time to clear the queues that have built up:

My colleague Ben Quinn is at Heathrow speaking to passengers disrupted by the computer glitch. Airport staff are working hard but patience among passengers is beginning to fray, he says.

From the Guardian’s transport correspondent, Gwyn Topham:

Air traffic control firm Nats are still investigating the cause but now confirm that contrary to earlier rumours it was not due to a power outage.

Press Association notes that the air traffic control centre in Swanwick, Hampshire, where the technical issue occurred, has had a chequered history since opening in 2002:

Originally the cost was going to be 132 million and the move-in date was scheduled for 1997. But various software problems that had to be overcome meant the cost rocketed and the switch over from West Drayton was constantly delayed. Eventually Swanwick opened in January 2002 - at a cost of 700 million.

Major US airlines and airports have faced minimal affects from the London air restrictions, reports my colleague Amanda Holpuch.

Most flights are due to leave for London later today and flights from London took off before the issue emerged.

American Airlines said it had two flights affected by the air closure. Flights AA47 to Chicago and AA81 to Dallas Fort Worth are being held on the gate at Heathrow. The airline said it would continue to provide information as it becomes available.

Boston Logan International airport spokesperson Matthew Brelis told The Guardian that the closure had not affected flights at the airport. “The morning flight had already departed and the flights that left are coming in,” Brelis said.

Delta Airlines advised passengers bound for London to check flight status through the day. “Delta is in close contact with NATS, Euro Control, the FAA and other aviation stakeholders as the London airspace issue continues to evolve,” said Delta spokesperson Morgan Durrant, before the airspace re-opened.

United Airlines said that operations are returning to normal and that none of its inbound flights were affected. “We only had three outbound flights that were minimally affected,” the company said.

Los Angeles International Airport spokesperson Amanda Parsons advised passengers to check with their airlines if they have a flight scheduled today, and not exclusively for passengers headed to London. Southern California is experiencing powerful storms today which could affect all flights out of LAX. She said the two flights scheduled to arrive from London today have already landed and the two flights departing to London are scheduled for later tonight.

Fewer than half a dozen flights are affected at Chicago O’Hare International Airport.

This picture shows the huge queue of flights waiting to land at Heathrow:

My colleague Samuel Gibbs explains why a computer glitch at air traffic control firm Nats might affect London flights:

Nats handles the flights of planes over Britain and in particularly London, the UK’s busiest airspace. When running at full capacity, the computer systems of Nats allow planes to fly close together in and guides them into alignment with the runways of airports. At that point the planes are handed over to the air traffic controllers of the individual airport, be that Heathrow or Gatwick for final approach and landing

When the technical issue reduced the Nats computer system capacity at Swanwick, it affected the ability of Nats to handle aircraft in such tight flying patterns. That caused the delays, where planes where directed to wait before taking off or be held in holding patterns above airports or in other originating countries.

Flights are now able to leave Gatwick airport:

The technical issue might have been fixed, but the travel disruption will be felt for hours to come.

Manchester airport is warning passengers to expect delays – and people sat in terminals across Europe are tweeting about the chaos:

Shortly after posting this tweet, this Twitter user said that he and fellow passengers were told to get off the plane and check in again:

The exact nature of the “technical incident” causing the airspace restriction is not yet known, but here’s what we know so far, by Guardian technology reporter Samuel Gibbs:

• A computer system failure has reduced the capacity of the National Air Traffic Services (Nats) air traffic control centre in Swanwick for handling aeroplanes over Britain. The “technical incident” has forced Nats to reduce the number of planes it can support particularly over London.
• Nats operates both the London Area Control Centre (LACC), which handles air traffic passing over London, and the London Terminal Control Centre (LTCC), which controls aircraft landing at or taking off from London’s six airports, including Heathrow and Gatwick. Both centres are based in Swanwick.

There’s bound to be a significant knock-on effect from the London airspace restrictions, but my Guardian US colleague Amanda Holpuch reports that flights to a major Washington DC airport have been unaffected:

Washington Dulles International Airport in Virginia, just outside of Washington DC, is not seeing an impact from the air closure.

“Our afternoon bank of flights from London are already in the air heading to us and our afternoon bank taking off back for London or early evening are not scheduled to take off for a number of hours,” said Chris Paolino, spokesperson for Dulles and Reagan national airport. “At this time we’re not seeing any impact. Obviously, if this continues or expands, there may be, but right now no impact.”

It might be causing travel misery for some, but these travelers are making the most of being grounded at Heathrow:

They might want to make that drink last though, as Heathrow is currently not expected to let any fights take off or land until 7pm.

This graphic shows how the restricted airspace is affecting flights around Heathrow in west London:

You can see a live version of that graphic here.

Gatwick airport is also affected by the major computer glitch, reports Sky News:

It was initially reported that London airspace had been closed, but now Nats says it is only restricted.

It says:

NATS can confirm that a technical problem has been reported at Swanwick air traffic control centre.

UK airspace has not been closed, but airspace capacity has been restricted in order to manage the situation. We apologise for any delays and our incident response team has been mobilised.

Every possible action is being taken to assist in resolving the situation and to confirm the details.

Further information will be released as it becomes available.

Air traffic control firm Nats has just tweeted this:

Flights have been thrown into chaos after a technical problem at the southern England headquarters of air traffic control company Nats.

Air traffic management company Eurocontrol said there had been “a failure” at the centre and airports would not be able to accept any traffic until 6pm UK time.