The doors on both engines of the flight that made an emergency landing at Heathrow last week had been left unlatched, investigators have found.
Air accident experts said the coverings - the fan cowl doors - broke off and punctured the right engine's fuel pipe, damaging the aircraft's systems.
A fire then broke out in the right engine of the Airbus A319 British Airways flight to Oslo.
The jet landed safely and its 75 passengers and crew were evacuated.
The aeroplane returned to Heathrow soon after taking off last Friday when black smoke was seen coming from an engine.
The findings were made in an interim report by the Air Accidents Investigation Branch (AAIB), which is examining the cause of the emergency.
'Appropriate initial action'
The fire in the right engine broke out as the flight prepared to land, but the left engine was unaffected, the AAIB report said.
"Subsequent investigation revealed that the fan cowl doors on both engines were left unlatched during maintenance and this was not identified prior to aircraft departure," the report added.
Emergency services attend plane Passengers and crew were evacuated from the plane via safety chutes
Keith Williams, BA chief executive, said: "We continue to co-operate fully with the investigation team and can confirm that appropriate initial action has already been taken in accordance with the AAIB's safety recommendation to Airbus."
Mr Williams said BA could not discuss any details while the inquiry continued, but added: "We commend the professionalism of the flight crew for the safe landing of the plane, and the cabin crew and pilots for its safe evacuation."
BBC transport correspondent Richard Westcott said British Airways had confirmed that two different engineers would normally check whether a plane's engine covers had been shut before take-off.
"One checks, the other double-checks - that's clearly not happened in this case," he said.
Mr Westcott added that BA's mechanics were all staff of the airline, which would not say if anyone had been suspended.
'Slight bump'
The aircraft underwent normal overnight maintenance which included opening the cowl doors and checking the oil levels, the report said.
Nothing unusual was noted during the checks before the flight took off, but the cowl doors would have been "difficult to see unless crouched down so that the bottom of the engine is clearly visible".
As the plane took off the 50-year-old pilot reported feeling a "slight bump" and thought the aircraft had run over a light on the runway.
Eyewitness Steve Fryer: "Flames were coming out of it"
Air traffic controllers alerted the pilot about the flight leaving some debris on the runway, which later turned out to be the detached cowl doors.
When the crew realised the doors had broken off, causing a "significant fuel leak" and affecting the hydraulic system, the pilot decided to return to the airport.
The cowl doors also damaged the tyres, landing gears and the hydraulic brake pipe.
The right engine was "extensively damaged" in the fire, the report said.
Last July Airbus said there had been 32 reported fan cowl door detachment events, but none of the cases resulted in a fire.
"The source of ignition that led to the in-flight fire is still under investigation," the report said.
The AAIB has asked Airbus to inform operators about the 24 May emergency and recommended it "reiterates the importance of verifying that the fan cowl doors are latched prior to flight by visually checking the position of the latches".
