Scotland Yard said the incident involving flight BA038 from Beijing was not terror-related. A chartered BA Boeing 747 due to fly Prime Minister Gordon Brown to Beijing for and official visit was held up as a result of the incident.

As passengers spoke of a "lucky escape" via the plane's emergency chutes, BA declined to comment on a BBC report that the jet had lost power and avionics in the approach to Heathrow.

"We can confirm that flight BA038, a Boeing 777 arriving from Beijing, carried out an emergency landing at Heathrow Airport today at 12:42 [p.m. local time]," the airport authority says on its website. "Heathrow’s emergency services attended the scene and passengers were immediately evacuated and taken to a reception center at the airport. The Air Accidents Investigation Branch is attending the scene."

With its undercarriage already rolled out, the plane came down several hundred metres short of the runway, and within metres of a perimeter fence separating the airport from a busy perimeter road.

Passengers said there been no warning but there was "no panic" on board as the crew conducted an orderly evacuation.

Officials said delays were expected after one of Heathrow's two runways was closed. An air exclusion zone was imposed to help regulate traffic at one of the world's busiest airports.

BA chief Willie Walsh, while praising the crew for doing an "excellent job," declined to comment on the possible cause of the accident, which is being to be probed by the Air Accidents Investigation Branch (AAIB).

“It would be inappropriate for me to speculate as to the likely cause of this incident,” British Airways Chief Executive Officer Willie Walsh said in a statement.