AN RAF Nimrod spy plane that crashed in Afghanistan last month killing all 14 men on board broke up in mid-air after a fractured fuel line set off explosions, an official inquiry has found.

The Nimrod MR2, which was providing intelligence for a Nato operation against Taliban fighters west of Kandahar, had just refuelled at 22,000ft.

Sources have disclosed that an RAF board of inquiry’s preliminary report has found that the fuel line fractured either as the Nimrod MR2 was being refuelled or shortly afterwards.

Fuel and vapour that leaked into the bottom of the fuselage then caught fire, possibly because of an electrical fault. Enemy fire has been ruled out as a cause.

The wreckage was so damaged that the investigators have not found direct evidence of the fire. They have had to rely on the pilot’s transmissions before the explosions and records of work done on the plane.

The conclusions will lead to serious concern about the RAF’s Nimrod fleet given that the aircraft have been flying since 1969 with a big upgrade started four years ago. The refurbishment was delayed by a series of problems — for instance, the wings BAE Systems designed would not fit the fuselage. The revamped aircraft will not return to service until 2009.

Nimrod XV230 (the lost jet’s call sign) was providing electronic surveillance for British special forces taking part in Operation Medusa against the Taliban.

The pilot reported a fire on board and was trying to land at Kandahar nearby when a series of explosions led to the aircraft breaking up in mid-air.

Debris was scattered over an area more than a mile long, according to the inquiry, which is taking place at the aircraft’s station, RAF Kinloss in Morayshire.

The crash was the biggest single loss of life for the services since the Falklands war in 1982 and the biggest for the RAF since a special forces C-130 Hercules was shot down by insurgents in Iraq two years ago, killing all 10 men on board.

Soldiers from the RAF Regiment secured the Nimrod crash site and recovered the bodies, but the destruction was so extensive they were unable to recover many aircraft parts.

The crew of 14 consisted of 12 RAF men from 120 Squadron and two special forces signallers relaying intelligence gathered to troops on the ground. Within 24 hours investigators arrived from Britain, but the hard evidence was limited and they had to rely on photographs, maintenance documents and recorded cockpit transmissions.

They concluded that the fuel was safely delivered from the tanker via the refuelling probe above the Nimrod’s cockpit.

It was then pumped down the fuel line to where a computerised control box steadies its flow into the tanks in the wings.

The fuel line that fractured is believed to be somewhere near the control unit in the bottom of the fuselage. The transmissions show the pilot reported a fire, but was apparently unaware of the fuel leak that had caused it.

The full report is some months away but a source close to the inquiry said: “It is clear that there was a fire and perhaps one or more explosions aboard the Nimrod and these factors indicate a fuel source as the cause of the tragedy.”

The Nimrod fleet has not been grounded but recommendations for flight safety will be made by the Defence Aviation Safety Centre at RAF Bentley Priory, northwest London.

“This is a very difficult investigation,” a senior defence source said. “Every aspect of the aircraft’s paperwork history has been checked . . . and all other MR2s have been checked out as a matter of standard practice.”

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