An SAS sniper jailed for illegally possessing a pistol and ammunition is hoping to get an 18-month sentence quashed.
Lawyers representing Sergeant Danny Nightingale, 37, are due to ask appeal judges to analyse the case at a hearing in London.
Sgt Nightingale was sentenced to 18 months' military detention earlier this month by a judge sitting in a military court.
He admitted illegally possessing a Glock 9mm pistol and more than 300 rounds of ammunition at a court martial.
Three judges - the Lord Chief Justice, Lord Judge, Mr Justice Fulford and Mr Justice Bean - are scheduled to hear legal argument at a hearing in the Court Martial Appeal Court.
Sgt Nightingale, a father of two, is being held in a military centre in Colchester, Essex. He told the court martial that the gun was a gift from Iraqi soldiers he had helped train.
Sgt Nightingale, who has suffered medical problems affecting his memory, said he did not remember having it.
His wife Sally, 38, who lives near Crewe, Cheshire, said more than 85,000 people have signed a petition calling for him to be freed.
Mrs Nightingale told ITV Daybreak that, although more than 100,000 people had now signed the petition, her husband remained cautious about Thursday's appeal hearing.
She said: "He doesn't want to get his hopes up too high because we were very positive about the court martial and we got such devastating news there, so he has to be really careful about how excited he is."