When the airline's engineers serviced and reassembled the doomed plane's left undercarriage four days before the crash on 25 July, they left out a vital component - the 'spacer' - which is designed to keep the wheels in a straight alignment.

Senior Concorde pilots and other industry sources say this omission appears to have caused the aircraft to veer wildly towards the edge of the runway, making it behave uncontrollably like a supermarket trolley with a jammed wheel. This forced the captain to try to take off when the plane was travelling 11 knots below the recommended minimum speed of 199 knots.

The plane had already developed a fire beneath its left-hand wing, caused when a tyre burst after hitting a strip of metal on the runway. However, by being forced to take off too slowly, the crew faced a much more difficult task in trying to rescue the aircraft. Had they not done so, they would have ploughed into the grass at the side of the runway, or hit a 747 waiting on a taxiway, which was carrying the French President, Jacques Chirac.

The French air accident investigation bureau, the BEA, denies the missing spacer caused the plane to veer, blaming this on a loss of thrust in the left-hand engines, which resulted from the fire. Senior pilots inter viewed by The Observer question this conclusion, saying that such a loss of power before the accident would not produce a sideways movement of this kind.

John Hutchinson, a BA Concorde captain for 15 years, said to lose power from an engine was normally 'no big deal', and that any sideways 'yaw' would be 'easily containable'. Skidmarks on the runway support the theory that veer was caused by the missing spacer, he added.

The Observer investigation also discloses that when the plane began to move down the runway, it was several tonnes above its maximum recommended weight. This made the attempted rescue still more difficult. After takeoff, the crew shut down one of the engines, contrary to established procedures, limiting their options further.

The Civil Aviation Authority grounded BA's seven Concordes three weeks after the accident, saying the accident had been caused solely by debris from the burst tyre. BA is now spending £30 million in an effort to get the plane flying commercially again later this year, by equipping it with kevlar fuel tank linings and new radial tyres.

Last week The Observer tried repeatedly to ask Air France to comment. But its spokespeople said that it would not any answer questions about the crash.