Deaths of 10 UK personnel in Iraq would not definitely have been prevented had a safety device been fitted to their plane, says the Ministry of Defence.
The 10 deaths marked the single biggest UK loss of life in Iraq
The Hercules crashed in January 2005 after being hit by ground fire, which caused a fuel tank explosion.
RAF papers obtained by the BBC suggest pilots had as early as 2002 requested explosive-suppressant foam devices.
The MoD said probes had found that such devices would not definitely have changed the outcome of the attack.
The crash was the single largest loss of British life in Iraq since military action began in 2003.
The explosive-suppressant foam stops fuel tanks from exploding when pierced by bullets. One US plane fitted with it managed to land safely after being shot 19 times in Iraq.
An MoD statement acknowledged an RAF board of inquiry's finding that the lack of a fuel tank "inerting system" had been among the "contributing factors" to the crash.
But the statement went on: "Whilst such a system may have prevented an explosion in the wing, the board concluded that it would not necessarily have prevented a fire.
"Although such a system would have increased the probability of the aircraft's survival, neither the board of inquiry nor the independent senior air accident investigator concluded that it would definitely have changed the tragic outcome."
An internal RAF document obtained by BBC Radio 4's Today programme suggested requests were made for the explosive-suppressant foam at least as early as 2002.
It read: "Urgent operational requests for all Hercules aircraft should continue to be actively pursued. Specifically, all aircraft should be fitted with fire suppressants in fuel tanks."
Today was told a US pilot refused to fly in a British plane because of concerns he had about safety.
The foam has been in use in US Hercules aircraft since the Vietnam War.
Former RAF Hercules pilot Nigel Gilbert, who trained with the pilot killed in the attack, said: "I believe the probability is that the crew would have survived the attack if the aircraft had explosive-suppressant foam in the fuel tanks.
"The crew was so good they could have even put it down in a road or put their landing gear up and landed it straight ahead in the desert. It was as flat as a pancake."
The MoD said none of its planes in Iraq or Afghanistan had foam, but some would be fitted soon.
It said the planes facing the highest risk of attack would be fitted with the foam.
Michael Moore, foreign affairs spokesman for the Liberal Democrats, said it was "scandalous" that only some of the planes were to be fitted with the foam while dangerous deployments were ongoing in Afghanistan and Iraq.
Shadow defence secretary Liam Fox said the cost would only be about £275,000 plus £50,000 per aircraft.
"If the American and Australian governments are fully protecting their servicemen and women, it's a disgrace we can't," he said.
Campaigners say they will sue ministers for corporate manslaughter if any more lives are lost because of a lack of protective equipment.
The attack happened on 30 January 2005, downing a Hercules travelling from Baghdad to Balad.