The engine doors punctured the airframe and some aircraft systems after coming loose and the flight crew decided to return to Heathrow.
On the approach to land an external fire developed on the right engine, although the left engine performing normally throughout the flight.
The emergency services attended and extinguished the fire after the plane had safely landed.
The 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.