The most highly-decorated Marine to serve in Iraq and Afghanistan has defended a soldier who murdered a Taliban insurgent – saying “the only good militant is a dead militant”.

Former Lance Corporal Matt Croucher, who was awarded the George Cross after diving on an exploding enemy grenade in Afghanistan to protect his comrades, served alongside the sergeant, who can only be identified as Marine A, who will be sentenced for the crime this week.

Speaking exclusively to the Post, Mr Croucher described as Marine A “a friend, colleague and leader of men”. He urged leniency and a sentence that reflects the pressures faced by frontline soldiers risking life and limb in one of the world’s most brutal war zones.

The killing in Helmand province in September 2011 was caught on helmet camera and film clips seen in the court showed him saying “shuffle off this mortal coil”.

However, Mr Croucher said: “He is a professional and well respected Marine who I have respect for as a friend, colleague and leader of men – ‘is he a ‘cold blooded murderer’? No.

“I have stood next to Marine A numerous times when we have taken enemy prisoners in the past –­ which leads me to think this incident was in the extremes of personal conditions, whether physically, mentally or both. In the heat of an extreme situation, did he make a conscious error?”

He added: “Honestly, I can’t say yes or no – even with my experiences I can’t put myself in his shoes, feel what he and the other Marines were feeling at that moment in time. From being in similar situations however, my feeling is, the only good militant is a dead militant – they have no moral compass, they suppress the Afghan nationals with threats and violence, have a hatred for Britain, our values and if given half an opportunity would wreak terrorist havoc on British soil.”

A military board which includes Royal Marine officers decide the minimum term in prison for Marine A on December 6.

The marines were on patrol in Helmand Province in September 2011 when they discovered the insurgent lying seriously injured in a field following an attack by an Apache helicopter. Marine A was convicted after footage emerged of him shooting the insurgent at close range in the centre of his chest.

The serviceman, known as Marine A, was found guilty of murder following a two-week court martial and faces a mandatory life term. Two others, known only as Marines B and C, were cleared of the same charge.

Mr Croucher said the reality of being on the frontline in war-torn Afghanistan should be taken into account in sentencing Marine A.

“To wake up every morning uncertain of what the day ahead will bring – except the certainty of a determined enemy intent on killing or maiming you and your friends, in any underhand, sneaky way, shape or form they can, is the most challenging, stressful and dangerous profession I will ever experience,” he said.

“It’s an experience, no film or book can truly portray – you have to physically live and breathe it to understand the magnitude, dangers and challenges of fighting in a war.

“Get it wrong, you’re dead or worse still your colleague – Afghanistan is the ultimate physical and mental challenge.”

He added: “We, as Marines in a war zone, haven’t the luxuries of worrying about tax rises, energy bills, the winter weather or who is going to win X ­Factor. Our sole focus is to stay alive, accomplish our mission as best as we can and return home in one piece to our loved ones – and that we do with extreme pride and professionalism.”

Mr Croucher also painted a dark picture of the Taliban and the brutality shown towards both enemy soldiers and Afghan civilians.

“For those that have seen intense close-­quarter combat with the Taliban there is no equal,” he said. “We are dealing with an enemy which has no morals, which indiscriminately litters the Afghan land with IEDs and mines intent on killing or maiming British soldiers. However they often injure and kill civilian adults and children too.”

Having served and fought with Marine A in operations in both Iraq and Afghanistan, Mr Croucher said he was a soldier he knew well and had the highest regard for but could only speculate on the circumstances surrounding the shooting of the insurgent.

Mr Croucher added: “I do ask people to look outside the box and not have a narrow-­minded view on who, how and why Marine A did what he did.

“Friends and colleagues being killed and injured on a daily basis imposes an abnormal mindset and extreme emotions – it goes well beyond ‘just business’ and is a very personal affair for which each combat soldier deals with differently.”

Mr Croucher’s views echo those of distinguished former Royal Marines general, Major-General Julian Thompson, who also came to the defence of Marine A.

Maj Gen Thompson, who led 3 Commando Brigade during the Falklands War, said a five-year prison term would be more suitable than life imprisonment.