Ten servicemen died when a Hercules came down in Iraq
An RAF Hercules plane which crashed in Iraq killing 10 British servicemen in January was hit by ground-to-air fire.
An RAF Board of Inquiry found that was the most likely cause of the biggest single loss of British forces in Iraq.
The plane was more vulnerable as it was flying low, something now being avoided "whenever possible", Defence Secretary John Reid said.
At the time a video surfaced showing burning wreckage, claimed by insurgents to have been the shot down Hercules.
The aircraft took off at 1622 GMT on 30 January from Baghdad airport on a routine flight to the massive US base at Balad - on the day Iraq held historic elections.
The aircraft reported a fire six minutes after take-off from Baghdad. It was reported missing 24 minutes later and the wreckage was found by US helicopters 25 minutes later, about 40km (25 miles) north west of Baghdad.
Flt Lt David Stead
Flt Lt Andrew Smith
Flt Lt Paul Pardoel
Master Engineer Gary Nicholson
Chief Technician Richard Brown
Flt Sgt Mark Gibson
Sgt Robert O'Connor
Cpl David Williams
Sqn Ldr Patrick Marshall
Acting L/Cpl Steven Jones
The report said: "The board has determined that the aircraft crashed as a result of hostile ground-to-air fire which caused an explosion in the right hand wing fuel tank.
"This explosion caused the outboard section of the wing to separate from the rest of the wing at which point the aircraft immediately became uncontrollable."
Arabic TV al-Jazeera later aired a video reported to have come from insurgents who claimed they had shot down the plane.
Its authenticity was never established.
Mr Reid said the investigation board had made a number of recommendations, some of which had been acted on already.
The fitting of a fuel tank "inerting system", which replaces oxygen with an inert gas like nitrogen to avoid explosions, was being investigated "as a matter of urgency".
The lack of such a system - used on many military planes for decades - was considered a contributory factor, with an explosive fuel-air mix developing in the wing fuel tank.
Also, there had been two attacks on US helicopters in roughly the same area on the day the Hercules crashed, but this information had not been passed on to the crew.
The board said the lack of intelligence was a factor in the crash and recommended a more reliable and timely system to keep air crews updated.
The nature of weapon used in the attack was not being disclosed for security reasons, Mr Reid added.
He said there had been 800 cases of ground-to-air fire against British aircraft in the 18 months before the incident.
"There was a degree of perchance or [bad] luck on this occasion," he added.
Low flying not banned
Mr Reid praised the crew on the Hercules saying there was nothing they could have done once the plane was hit.
In reply to a question by shadow defence secretary Liam Fox, Mr Reid said the types of weapons used in the attack were "widely used" in Iraq.
Low level Hercules C130 flying in daylight was now being avoided "whenever possible".
Mr Reid said the tactic of low flying during daylight was not going to be prohibited, to allow military personnel to be able to choose the safest and most appropriate operational action.