The report said the BA plane had undergone scheduled maintenance the night before the incident, which required opening the fan cowl doors on both engines to check oil levels.
The report noted that the plane manufacturer Airbus had recommended airlines strictly adhere to maintenance standards following previous instances of fan cowl door separation on the A320 "family" of planes, which include the A319 in last week's incident.
The report said that last July Airbus said there had been 32 reported fan cowl door detachments.
On some occasions, significant damage was caused to the aircraft but none of those events resulted in a fire.
"The source of ignition that led to the in-flight fire is still under investigation," the AAIB said.
Engine-covering doors on both engines on the smoke-trailing British Airways plane in last week's Heathrow landing drama had been left unlatched during maintenance, an interim report by the UK Air Accidents Investigation Branch said today.
The British Airways plane in last week's dramatic emergency at Heathrow landed after pilots shut down one engine and the other engine was on fire, according to an official American accident investigation team.
Engine cowls, or coverings, on both the US-made engines of the Oslo-bound Airbus A319 came off and fell on the runway as the plane took off for Oslo last Friday morning, said the US National Transport Safety Board (NTSB).
The pilots reported that they shut down one engine, there was a fuel leak, and that they were returning, the NTSB said on its website .
British Airways (BA) has said that it will not pay compensation to passengers after flights were delayed and some cancelled after the emergency landing of one of its jets at Heathrow Airport last week.
A ruling by the European Court of Justice means that passengers whose journeys are delayed by more than three hours are entitled to compensation.
But the airline has claimed that it is not responsible for the £214 per passenger payment.
BA said the closure of the airport was an "extraordinary circumstance", and therefore it was only liable for providing food and drink, and hotel accommodation where necessary.
About 200 flights were cancelled following the closure of Heathrow on Friday, the majority of which were operated by British Airways (BA).
British Airways has said it will not pay compensation to passengers whose flights were delayed or cancelled following the emergency landing of one of its jets at Heathrow Airport last Friday.
Passengers whose journeys are delayed by more than three hours are entitled to compensation following a European Court of Justice ruling.
About 200 flights were cancelled following the closure of Heathrow on Friday, the majority of which were operated by British Airways.
But BA has insisted it is not responsible for £214 per passenger under the EU261 ruling.
It said the closure of the airport was an "extraordinary circumstance", and therefore it was only liable for providing food and drink, and hotel accommodation where necessary.
A BA spokesman said:
"We are very sorry for the disruption thousands of our customers experienced on Friday following the severe air space restrictions after Heathrow was closed for a short while and one of the two runways at the airport was shut for several hours."
He added: "Heathrow operates at maximum capacity and is the world's busiest two-runway airport.
This type of incident at Heathrow, regardless of which airline it involves, will always lead to large numbers of cancellations and delays for British Airways as we are the home-based carrier with 50% of the flights at the airport.
"We did all we could to offer duty of care including giving out refreshments and hotel rooms during Friday.
We do not believe we are liable for further EU261 compensation payments as the closure or restriction of airspace, which led to the cancellations and delays, is defined as an extraordinary circumstance."
A full report by the Air Accidents Investigation Branch (AAIB) could be some time away, but the branch could publish an interim report laying out the basic facts quite soon.
The AAIB will take into account not only crew, passenger and eyewitness statements but also data from the flight recorder.
Investigators will look into reports that part of the left-hand engine casing, which was shown to be off after the plane landed, actually detached on take-off.
One passenger told the online Daily Mail today: "'As the plane started to gather speed on the runway for take-off, the engine casing came loose and started flapping and at the point of take-off it snapped off with a loud bang."
"On the descent back into Heathrow the right engine burst into flames, creating an orange glow visible throughout the cabin. People were relatively calm until the engine blew and then started to panic."
Heathrow is now fully operational but experiencing some delays. The airport advises passengers to check with airlines before travelling to the airport.
British Airways gives the following advise to its passengers:
If your flight has been cancelled, please try rebooking on http://t.co/oRKt3apWw6
We're experiencing a high volume of calls. Please only contact us if your flight has been cancelled & you're unable to change it online
If you booked your flight or holiday with a travel agent, please contact them directly to change
Pictures taken from inside the plane in today's incident showed an inspection cover loose on the left-hand engine.
Aviation expert David Learmount said:
"This cover is to a plane what a bonnet is to a car. It should not have been open. Something caused it to be dislodged.
Pictures of the plane flying with smoke coming from it indicate that the aircraft was being powered entirely by the left-hand engine. Most likely there was external damage to both engines.
Damage of this kind is consistent with a bird strike although at this stage we just don't what happened."
British Airways said it had cancelled all its short-haul flights in and out of Heathrow until 4pm today following the emergency landing earlier.