The first few missiles deflected the defensive systems on the Hercules from the special forces flight of 47 Squadron, allowing the others to get through, the sources said.

Investigators who examined the wreckage near the town of al-Taji, 25 miles northwest of Baghdad, found at least five points where missiles hit the aircraft on January 30.

They are believed to have concluded that as many as six missiles and rockets, possibly more, were aimed at the aircraft and that the height at which it was flying gave the pilot no room for manoeuvre to avoid all of them. The findings will be revealed in a RAF board of inquiry report due to be published next month.

On Friday John Reid, the defence secretary, was among 1,600 people who attended a memorial service for the men in Salisbury Cathedral.

The crash, on the day Iraqis took part in their first democratic election, was the worst loss of life suffered by British forces in Iraq since the war began in March 2003.

British officials originally thought the loss had been much worse. Only a few hours earlier, the aircraft, designated XV179, had taken about 50 men from G Squadron, 22 SAS Regiment, down to Baghdad from the US base at Balad.

The SAS had been using Balad as a forward operating base for missions inside the Sunni triangle, the heart of the Iraqi insurgency.

Although insurgents had previously shot down several American helicopters, using anything from machinegun fire to rocket-propelled grenades (RPGs) and even anti-tank missiles, they had not destroyed a fixed-wing aircraft.

A group of insurgents from a group called the Green Brigade, part of the 1920 Revolution Brigade of the Islamic National Resistance, is thought to have prepared an ambush intended for the return of an American plane that had flown low over the area the previous day.

They might have been tipped off about XV179’s takeoff from Baghdad by colleagues watching the airport and reporting each aircraft that took off.

The Hercules aircraft, piloted by Flight Lieutenant David “Steady” Stead, departed at 4.22pm. Among the men in the back were some of the RAF’s most experienced special forces aircrew including Flight Sergeant Mark “Gibbo” Gibson, 34, the loadmaster, and Master Engineer Gary Nicholson, 42, with a career in 47 Squadron that dated to 1984 and included service during the 1991 Gulf war and a series of operations in the Balkans.

Of the others sat in the aircraft hold, Corporal Dave Williams, a veteran of 17 years in the RAF, was there to ensure all the survival equipment was in place, a crucial task on a special forces aircraft.

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