Married father-of-two Nightingale, who’s served for 17 years, pleaded guilty to illegally possessing the gun, which was mailed to him by mates after he had to return from Iraq to the UK to arrange the funerals of fallen comrades.

The Telegraph reports  he had no memory of owning the gun, telling the trial he suffered severe memory loss because of a brain injury from when he fell into a coma while trekking in Brazil in 2009.

Four senior officers including Colonel Richard Williams (Sgt Nightingale’s commanding officer in Iraq), Colonel Tim Collins, Andy McNab and Chris Ryan sent a letter to Mr Cameron, published by The Sun.

 “We say this prosecution should never have happened. Furthermore, we say he was bullied into a guilty plea. And lastly, we say the custodial sentence is completely disproportionate to the alleged crime.

“We say he is the victim of a shameful travesty of justice and we demand immediate action.

“Compare this man’s case to that of Abu Qatada (the Islamic extremist who won an appeal against extradition to Jordan this week to face terrorism charges) and see if it is fair.

“We call on you to exercise leadership and judgment and to release Sgt Nightingale today on licence, so that this case can be reviewed with the utmost haste.”

After two of Sgt Nightingale’s closest friends died in a helicopter crash in Iraq in 2007, he accompanied their bodies to Britain, leaving colleagues to pack up his gear, which included the pistol. The gun was found during an unrelated police search in 2011.

He planned fight the charge but pleaded guilty to avoid a possible five-year sentence should he lose after pleading not guilty. His lawyer, Simon McKay, said he’ll appeal the sentence.

A Ministry of Defence spokesman said: “We do not ordinarily comment on UK special forces for very good reasons. It would also be wrong to comment on the process, findings, convictions or sentences of a court that may be subject to appeal. It is for the courts, and the courts alone, to determine the guilt or otherwise of any person accused of an offence.”

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