Seven Royal Marines were being questioned today about the killing of an insurgent in Afghanistan who is thought to have been captured and disarmed before he died.

In a major reputational blow to one of the most elite elements of Britain’s Armed Forces, the men were arrested yesterday by the Royal Military Police on suspicion of murder.

The rounding up of such a large number of Marines – a typical section is seven or eight men strong – was seen as an indication of the gravity of the situation.

“The thing that is serious is that this is a group of soldiers – that the group are complicit,” said Professor Michael Clarke, director of the Royal United Services Institute. “This is a nasty one I