Wiltshire Coroner David Masters has raised "concerns" about mine clearance in theatres of war after two Royal Marines serving in southern Afghanistan, including one from Somerset, were blown up by a makeshift device that metal detectors failed to find.

Marine David Marsh, 23, and Lieutenant John Thornton, 22, both of 40 Commando, were killed by an explosive near Kajaki, in Helmand Province, on March 30 last year.

Wiltshire coroner David Masters ruled that the pair were unlawfully killed - murdered - by unknown insurgents who planted the 7.5kg device.

Mr Masters said he was "concerned" that training on the metal detecting equipment was largely theoretical before troops got to the warzone.

He also noted that one of the two detection devices was being used at a lower level of sensitivity than was recommended, even though it was fully functional on that setting.

The coroner added that both metal detectors were in working order before the blast and there was no "certain cause" for the mine not being found.

He also stopped short of using his legal right to recommend changes to military procedure, during the inquest at Trowbridge Town Hall yesterday.

Lt Thornton, known as JT, from Ferndown, Dorset, joined the Royal Marines on August 31, 2004, and had previously served in Iraq. He is survived by his parents Linda and Peter, and his brothers Ian and Graham.

Marine Marsh, known as Dave, was from Sheffield, but had recently moved to Norton Fitzwarren, near Taunton, with his wife Claire and young daughter Molly.

He joined the Royal Marines in September 2002 and also left behind parents, Gary and Maxine, and sister Katie.