A distinguished Royal Marines general has called for leniency towards a fellow commando who executed an injured Taleban insurgent in Afghanistan

Prime Minister David Cameron said today that the conviction of a Royal Marine for murdering an injured Taliban insurgent in Afghanistan should not "besmirch" the organisation's proud history.

Speaking on the steps of Downing Street to a group of serving and former Royal Marines raising funds for the Commando 999 charity, he said that yesterday all thoughts were on the "appalling" court case, during which the marine was found guilty of executing the seriously wounded prisoner in Helmand Province two years ago.

He told the marines: "That in no way represents the spirit and the history of the Royal Marines, an outfit that has one of the proudest histories of any in the world.

"We should not let that single incident besmirch the incredible work the Royal Marines have done, not only over decades but over centuries."

The Prime Minister said the Royal Marines "leave the country in a far better state that when we found it", with there being "no functioning Afghan state" when they arrived in 2001.

He said: "I wanted to say a big thank you to the Royal Marines for the incredible work you have done in Afghanistan over the last decade.

"I believe we will be able to leave that country with our heads held high."

The serviceman, a sergeant known as Marine A, was found guilty of murder following a two-week court martial and faces a mandatory life term when he is sentenced next month.

Two others, known only as Marines B and C, were cleared of the same charge.

Meanwhile a distinguished Royal Marines general called for leniency towards Marine A.

Major General Julian Thompson, who led 3 Commando Brigade during the Falklands War, refused to condemn the marine, and told The Times that a five-year prison term would be more suitable than life imprisonment.

Maj Gen Thompson said the shorter prison term was more appropriate for a crime committed under the unique pressures of war.

He said that "obviously it was wrong and everyone in the Royal Marines is quite clear about that".

But he added: "The Royal Marines are a family and it feels as though a member of the family has transgressed.

"I am sad for the man who did it, in that he probably had a moment of stupidity. I feel for him as I would my own son who might do something stupid."

He said that accepting an enemy's surrender on the battlefield was "a very, very dangerous time", and told the Daily Mail: "I have no sympathy for the man who was killed but Marine A did the wrong thing by shooting him.

"But I'm not going to stand around bad-mouthing him. I won't condemn him. It is like a member of the family who has broken the law - you don't reject them, but you support them."

Speaking on BBC Radio 4's Today programme, he said that w hile the action was "totally unforgivable", it was important to understand the sort of pressure a battle-hardened soldier could find himself under.

Dismissing the argument that there should be less understanding for an experienced soldier doing such a thing, he said: "The more times you do tours in that filthy war, the more pressure there is on you.

"We don't know what sort of pressure he was under, however cool he may have sounded on that tape and how pre-meditated it may have sounded.

"He certainly shouldn't serve any less than five years, maybe 10."

He said he expected the Marine to be looked after by the military community.

"He shouldn't be treated as a pariah. He should not be kicked out of the community as it were. I hope and I'm sure the Marines will support him and particularly his family through the years ahead," he said.

"I don't mean that he should be kept in the Royal Marines; what I am saying is that he should not just be rejected and flung to the wolves as it were.

"I do not mean that he's forgiven and allowed back in. What I mean is that he shouldn't just be spat on and rejected."

Marine A was convicted after footage emerged of him shooting the insurgent at close range in the centre of his chest.

As the man convulsed on the ground, Marine A told him: "There you are. Shuffle off this mortal coil, you c***. It's nothing you wouldn't do to us."

He then turned to comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."

During the trial, the court heard the marines were on patrol in a "kinetic" area of Afghanistan on September 15, 2011 when they discovered the insurgent lying seriously injured in a field following an attack by an Apache helicopter.

Superiors were informed the man had died from wounds sustained in the gunship attack, in which 139 30mm anti-tank rounds were fired at him.

But a year later, footage of the murder - taken from a camera mounted on the helmet of Marine B - was discovered on a laptop by military police investigating unrelated matters.

The recording revealed a conversation between the marines before the insurgent was shot.

Marine A was overheard asking "Anyone want to give first aid to this idiot?" before Marine B replies loudly "No".

Marine C, standing over the insurgent pointing a pistol at his head, is heard asking Marine A if he should shoot the man in the head, a request which is refused as "that would be f****** obvious".

The marines pretend to give him first aid, he is kicked in the torso by servicemen and soon after shot by Marine A.

During the trial, David Perry QC, prosecuting, told the court martial: "It was not a killing in the heat and exercise of any armed conflict. The prosecution case is that it amounted to an execution, a field execution.

Marine A told the court martial he thought the insurgent was already dead when he shot him, and said he did so out of anger, calling it a "stupid, lack of self-control, (a) momentary lapse in my judgment".

Marine B admitted members of the patrol lied during interviews to protect their comrade.

"We all protected him by telling lies," Marine B said. "In my opinion, he had shot an alive, injured insurgent."

Marine A was remanded in custody to be sentenced on December 6. Marines B and C were freed and returned to normal duties.

After the verdict Brigadier Bill Dunham, Deputy Commandant General Royal Marines, called the incident "truly shocking and appalling". He said: "It is a matter of profound regret in this isolated incident that one marine failed to apply his training and discharge his responsibilities.

"What we have heard over the last two weeks is not consistent with the ethos, values and standards of the Royal Marines.

"It was a truly shocking and appalling aberration. It should not have happened and it should never happen again."

He added: "It is now for the Royal Marines to consider any impact from this case on the training given to our people as we seek to uphold the very highest standards that we constantly strive to instill and perpetuate."

Sir Mike Jackson, a former head of the Army, told The Times: "The fact that due process of law has been carried out I hope demonstrates very clearly that nobody is above the law."

Military commentator Colonel Mike Dewar said he was "horrified" by Marine A's actions, but echoed calls for clemency.

He told BBC Breakfast: "I think we do need to understand there are fine margins on the battlefield.

"These soldiers have had their officer killed - their platoon commander, a very fine young man - they had seen their friends brutalised, - lost legs - they had been under the pressures of battle for weeks and months.

"I think you have to understand this is a completely different environment to a cold-blooded murder in normal circumstances.

"This was murder - have no doubt about it, and I make no excuses - but I think society does have to make some exceptions for soldiers in these extraordinary circumstances."

First Sea Lord, Admiral Sir George Zambellas, was also present as the Prime Minister made his comments to the marines, and joined in an impromptu round of applause after he so publicly voiced his support.

The Prime Minister added that he believed his backing of the Royal Marines was a view shared by the nation as a whole, he told the marines.

A group of them had just completed a world record speed march attempt to commemorate the 350th birthday of the Royal Marines, carrying 40lb over a 26.2 mile course, and were congratulated by Mr Cameron.

Labour leader Ed Miliband said he did not believe the case had diminished the reputation of the armed forces.

"When any incident like that happens, it is tragic and it must be dealt with. But what is really important is to know and to realise what our armed forces are and who our armed forces are and they are brave, decent, honourable people who do the right thing for our country.

"I think that's what the British people know."

Asked about the case while campaigning in north London, he went on: "It doesn't change the esteem in which our armed forces are held.

"Our armed forces are held in the highest esteem by the British people and by people all around the world, including people in Afghanistan who have seen, over a period of 10 years, British armed forces making a real difference to them, trying to keep them safe and help them...have a future of peace and stability for their country.

"They have done an outstanding job of doing that and I think we owe them a huge thanks for it."

It was a question for the courts whether the Marine should be allowed to remain anonymous, he said.