Pilots prepared a Boeing 777 for take off from Heathrow airport unaware that a fire had broken out on the plane, an accident report has said.

Four-and-a-half minutes passed before the cockpit crew of the United Airlines' plane learned of the Feb. 2007 emergency from a tug driver.

He raised the alarm after seeing smoke, caused by an electrical fault, coming out of the aircraft's vents.

The crew responded by shutting down the right engine, taxiing to a stand and safely evacuating the 185 passengers.

But their lack of awareness of smoke may have delayed that action, the Air Accidents Investigation Branch (AAIB) report said.

The AAIB said there had been "significant smoke" but that the possibility of a fire "appeared to have been missed" by the crew.

The passengers had been told by the relief co-pilot there had been an electrical problem which had shut down some of the air conditioning and cooling ducts.

Moments before, a flight attendant had told the cockpit crew "an electrical smell was present in the passenger cabin."

The co-pilot and relief co-pilot had told her they had a problem with the equipment and the electrics but that there was no fire.

The incident came after the plane had pushed back from its stand and the engines were being started prior to takeoff.

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