Marine identified only as A is convicted of murdering wounded man in Helmand province in 2011, while two others are cleared

A Royal Marines sergeant faces life imprisonment after being found guilty of the cold-blooded battlefield murder of a seriously wounded Afghan insurgent.

Two other marines were cleared of murder by a court martial board – the military equivalent of a jury – and have been allowed to return to their unit.

The court martial in Bulford, Wiltshire, was told that the three marines, who can be identified only as Marines A, B and C, were part of a patrol tasked to deal with an insurgent hit in a helicopter attack in Helmand in September 2011.

After dragging him to the side of a field, Marine A shot him in the chest while he was still clearly alive. B and C were also accused of murder by dint of helping and encouraging A, but were cleared. They insisted they were shocked when A opened fire.

There were gasps from relatives in the public gallery as the verdicts were returned, with some holding each other for support.

Judge Blackett told Marine A: "Marine A, this court has found you guilty of murder. The mandatory sentence prescribed by law is imprisonment for life.

"This court now has to determine the minimum term you will serve before you are eligible for release. To help us do that, I am going to order that a report be prepared. I am adjourning this case until a later date.

"In the meantime, I direct that you are to be taken into custody."

Addressing the other two defendants, the judge added: "Marine B and Marine C, you have been found not guilty of murder and you are now free to return to your normal place of duties."

The verdict is a huge blow for the British services and historically significant. It is the first time a British serviceman or woman has been found guilty of murder during an overseas operation in modern times. The Royal Marines called it a "truly shocking and appalling aberration."

Marine A, an experienced marine in his 30s, will be sentenced next month after psychiatric reports have been produced.

The case has also been a test for the principle of open justice. The marines were afforded anonymity throughout because of the danger that their lives could be under threat from extremists if their identities were known.

A damning video that captured the moment of the murder – taken from a helmet camera worn by Marine B – was not released after the government argued it would be a recruitment gift for terrorists and could lead to "lone wolves" to attack British troops or citizens in revenge.

The prosecution alleged the incident amounted to an "execution" of a man who was hors de combat – out of the fight – and should have been treated with respect and dignity. Its case centred on a shocking video – captured on a head camera worn by Marine B – that showed the insurgent being dragged roughly from a field after he had been injured in an air strike from a British helicopter.

Members of the eight-man patrol are heard abusing him and laughing at him.

Marine C, the youngest of the accused men, is heard saying: "I'll put one in his head if you want." There is laughter and Marine A replies: "No, not in his head 'cause that'll be fucking obvious."

Bleeding and moaning, the man is dropped at the edge of the field in Helmand. C says: "Maybe we should pump one in his heart."

The prosecution claimed the marines waited for the Apache helicopter to leave the scene so their actions would not be seen. Marine A then leans over and shoots the insurgent in the chest with a pistol before telling him: "There you are, shuffle off this mortal coil, you cunt. It's nothing you wouldn't do to us."

A few moments later A is picked up telling colleagues: "Obviously this doesn't go anywhere fellas. I've just broken the Geneva convention." Marine B laughs and suggests that if the round was heard they should claim it was a warning shot.

The video was found by chance a year later when one clip was found on another marine's laptop during an unrelated investigation. Military SIB (special investigation branch) officers subsequently found a diary kept by Marine C in which he wrote he had been disappointed not to shoot the man.

The prosecution had been prepared to release the video – with the marines' and victim's faces pixelated – but the government successfully argued it would have been an ideal recruiting tool for extremists.

In addition the three marines were granted anonymity because the judge advocate general, Jeff Blackett, who oversaw the trial, agreed their lives would be at risk if their identities were known. At all times they were hidden from the press and public by screens.

The media, including the Guardian, have challenged the anonymity order. Before the verdict the judge said he would lift it whether the men were convicted or acquitted. But the men's barristers said they would appeal – and that their identities should remain protected in the meantime. The issue will now be considered by the court martial appeal court.

The suppression of the video has minimised the impact of the case, which would have received worldwide coverage if it had been released, but the trial has still been very damaging to the British military and in particularly to the Royal Marines.

All three men denied murder. During the trial Marine A accepted he had fired at the man but said he believed at the time that he was dead and he was shooting a corpse out of "frustration" at the end of a tough tour during which time the marines had lost many colleagues.

He said he believed he had broken the Geneva conventions by desecrating a prisoner's corpse rather than shooting dead a captive. But he also now accepts that the video appeared to show that the man was alive when he discharged his weapon.

Marines B and C said they had no idea that A had been going to shoot the man and denied they had encouraged or assisted him. Marine B admitted he had hatched the warning shot plan to cover up for a fellow marine but said his laughter had been a nervous response.

Marine C claimed that his offers to shoot the prisoner had been bravado and said his diary was a way of letting off steam rather than an accurate record of what happened.

After the hearing Brigadier Bill Dunham, the deputy commandant general Royal Marines, said: "Today saw a Royal Marine found guilty of the murder of an unknown male. The naval service respects the verdict, which was reached in full accordance with UK law – to which all service personnel are subject, and in the name of which many thousands of Royal Marines have fought the Taliban in Afghanistan since 2001.

"The naval service takes seriously its responsibilities to train its sailors and marines in the standards of conduct applicable to combat operations on land, at sea and in the air. 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.

"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.

"The Royal Marines deservedly have a worldwide reputation as one of the elite fighting forces. Our commandos go through one of the toughest training programmes to deploy to some of the harshest environments in the world. I am as proud of our reputation and achievements as I have been at any time during my 34 years of service and there is no clearer demonstration of the Royal Marines ethos than the success and sacrifices made in Afghanistan under the most demanding physical and mental conditions."