LONDON - The British Airways plane that crash-landed at Heathrow last week did not suffer a total power failure as it approached the airport, investigators said on Thursday.

The Air Accidents Investigation Branch said the Boeing 777's two engines failed to deliver extra thrust as it came into land, but did not cut out completely.

In a second preliminary report, investigators said they were still trying to work out what caused the lack of thrust during the flight's final moments.

"The engines both initially responded but after about 3 seconds the thrust of the right engine reduced," the report said. "Some eight seconds later the thrust reduced on the left engine to a similar level.

"The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust."

The reason for the lack of thrust is not yet known. The plane, which was flying on autopilot, was carrying enough fuel.

Investigators will now examine the system which takes the fuel from the tanks to the engines.

Eighteen passengers were slightly injured when BA Flight 38 from Beijing landed short of the runway last Thursday.

Its undercarriage was ripped off and its wings damaged after it only just cleared the perimeter fence.

Scores of flights were cancelled. Among the planes delayed was a flight carrying Prime Minister Gordon Brown on an official trip to India and China.