Five of the nine Royal Marines arrested in connection with the death of a Taliban fighter have been charged with murder in what is stated to be the first incident of its kind involving British troops in Afghanistan, reviving memories of their murky record in Iraq.

All belong to the 3 Commando Brigade, an elite corps, whose track record in Afghanistan, until now, had been rated high. The incident happened when they were serving in Helmand province last year. All have since returned to Britain.

The decision to charge them was taken by the Service Prosecuting Authority, an independent body that conducts prosecutions on behalf of the army. They are likely to be tried by court martial.

The Ministry of Defence said the incident followed an “engagement with an insurgent” but did not give details.

The arrests — initially seven, followed by two later — were made after the Royal Military Police, investigating a separate case, found suspicious footage on a soldier’s laptop. It reportedly showed a group of servicemen discussing whether an injured man lying on the ground should be given medical treatment. The man later died.

Defence Secretary Philip Hammond said the Ministry of Defence was “determined” that rules of engagement must be followed by British troops and any abuse would be “dealt with” according to the rules.

“Everybody serving in theatre knows the rules of engagement, they carry cards in their uniforms with the rules on them in case they should need to remind themselves,” he told the BBC.

Defence observers said the case could have a serious impact on the army’s reputation. “There is no doubt about it, this is politically damaging because we’re not talking about an individual case here,” said Professor Michael Clarke, director general of the Royal United Services Institute.

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