An Oxford coroner called today for the RAF's entire Nimrod patrol plane fleet to be grounded immediately, after ruling that an aircraft which crashed in Afghanistan, killing 14 servicemen, had never been airworthy.

Oxfordshire assistant deputy coroner Andrew Walker said that opportunities to spot inherent dangers on the ageing plane were missed.

He said a design fault which led to the plane exploding just minutes after undergoing air-to-air refuelling went unnoticed.

Mr Walker recorded narrative verdicts on the deaths of the 14 personnel killed in the explosion near Kandahar on September 2, 2006.

The families of the victims, who had sat in court throughout the three-week hearing, had heard the disaster was caused by fuel leaking into a dry bay and igniting on contact with a hot air pipe.

The fact that the fuel couplings were in the same compartment as a hot air pipe was a "fundamental design flaw" in the 37-year-old Nimrod fleet, the inquest was told.

A senior RAF officer admitted the aircraft should not have been passed safe to fly and said mistakes were made during a hazard assessment which could have identified the fault.

Mr Walker, in his conclusion to the inquest, praised the "bravery and courage" of the servicemen who died and the determination of the families in their "drive to uncover the truth".

He said: "I have no doubt that these fine men will never be forgotten and their loss will be keenly felt by their families, friends and our armed forces.

"The crew and passengers were not to know that this aircraft, like every other aircraft within the Nimrod fleet, was not airworthy.

"What is more, the aircraft was, in my judgment, never airworthy from the first release to service in 1969 to the point where the Nimrod XV 230 was lost."

The Nimrods are stationed at RAF Kinloss in the north-east of Scotland. Based on the old Comet airliner, they were originally designed as a maritime patrol plane, but are used in Afghanistan on intelligence-gathering surveillance missions.

A heavily modified version, the Nimrod MRA4, is due to enter service in 2010.