A formal investigation is under way to find out what led one of the safest aircraft in the world to crash land at Heathrow airport this afternoon.

Passengers aboard the BA38 from Beijing were reflecting on their lucky escape, after all 136 were safely evacuated when the stricken aircraft tore into the tarmac. Only three suffered minor injuries.

The stricken Boeing 777 reportedly approached the southern runway of Europe's busiest airport at an unusual angle, plunging beyond the runway.

It finally came to a halt 1000m from an aircraft carrying the Prime Minister and 25 senior business executives, including the Virgin boss Sir Richard Branson and the Olympic gold medallist, Dame Kelly Holmes, who were departing for a five-day visit to China and India.

It is not yet known what caused the emergency landing although one BA worker, who asked not to be named, said the captain told him the plane lost power without warning.

He told BBC Radio Five Live: "They just told me that the aircraft just shut down and everything. He lost all his power and everything and that was it. He glided it across, lifted the nose up, and managed to get it in, and it's gone on to the grass.

"He said to me he had no warning , absolutely nothing at all. It's just suddenly, boom, it's just lost everything.

"It's a miracle. The man deserves an absolute medal as big as a frying pan."

Neil Jones, a witness who has a general aviation pilot’s licence, said he had seen the plane making a “very, very unusual approach” to Heathrow. He said: “The engine noise sounded louder than normal and it was that that first attracted my attention.

“The aircraft was banking to the left and it was coming in very low over the surrounding houses. The plane was significantly lower than it would normally be.

“I could see the undercarriage was down and the wing flaps were down. I don’t know how many engines were working.

“You could see the pilot was desperate, trying to get the plane down. The aircraft hit the grass and there was a lot of dirt. The pilot was struggling to keep the plane straight. I think he did a great job.”

Paul Venter, who was on board flight BA38, said the aircraft hit trouble just as it was about to land.

“We had a good flight, we came in to land, I could hear the undercarriage come out and the next moment the plane just dropped,” he said.

“The wheels came out and went for touchdown, and the next moment we just dropped. I couldn’t tell you how far.

“When everything came to a standstill, I looked out of the window and the undercarriage was gone and the plane was on its belly.

“I didn’t speak to the pilot, but I saw him, and he looked very pale, but there was no communication in the cabin."

Mr Venter said that, after evacuating the plane, the passengers were met by police and medical staff and taken to the terminal building for treatment and security processing.

“You’ve got some pale faces here, but I would say the mood is good,” he said.

Six passengers were taken to the nearby Hillingdon Hospital with minor injuries, a London Ambulance spokeswoman said.

Jerome Ensink, a passenger who was evacuated from the plane, said: "I'm beginning to realise we have had a close call. I'm a bit shaken, but feeling quite lucky. There seems to have been no major injuries.

"Everything seemed like a normal landing, until we hit the tarmac very roughly, but it wasn't until we were coming off the grass, that we realised the entire undercarriage was missing. We landed on the grass about 100 ft from the runway.

"The oxygen mask were coming down and the neon lights came up."

Mr Ensink added that the incident would not prevent him from flying British Airways in the future.

"BA have been dealing with the situation very well," he said.

The incident left the airport, which has 67.7 million passengers passing through every year, without any fire cover, bringing air traffic to a standstill.

The Prime Minister's flight finally took off for China, after a 55-minute delay.

Television footage showed the plane stranded at the end of the runway, with all of its emergency chutes inflated. Two of the plane’s giant wheel units were ripped from the craft during the landing and were visible on grass near the runway.

Kieran Daly, from Flight International magazine, said he was shocked by the incident, given the aircraft, airline and airport involved.

“The 777 is pretty much state of the art at the moment, even though it has been around for quite a long time.

“It’s very difficult to understand what might have gone wrong.

“We’re talking about one of the most advanced aircraft in the world, operated by one of the most safety-conscious airlines in the world, flying into one of the safest airports in the world.

“It’s quite a surprise."

In a press conference this afternoon, Willie Walsh, the BA chief executive, said a formal investigation was being carried out by the air accident investigation branch of the Department of Transport. He said: "It would be inappropriate for me to speculate as to the likely cause of this incident today."

Mr Walsh went on to praise the efforts of the flight crew and the emergency services, who helped avert a much more serious disaster.

He said: “We are very proud of the way our crew safely evacuated all 136 passengers on board with only three minor injuries. "He added that there were three flight crew and 13 cabin crew on board."

The northern runway was closed for more than an hour after the aircraft made it's emergency landing at 12.42pm

The south runway remains closed, while the airport struggled to deal with the backlog of traffic.

Three flights were diverted to Stansted, four to Luton, and 12 to Gatwick.

Heathrow has two runways, serving four terminals. A fifth is currently under construction.

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