The case of an SAS sniper jailed for illegal possession of a weapon sparked a political split as attorney general Dominic Grieve turned down Philip Hammond's request to review the prosecution on Tuesday.
Sergeant Danny Nightingale's wife was left "disappointed" after Mr Grieve said it would be inappropriate for him to review the decision to prosecute.
Sally Nightingale took her husband's case to the attorney general without success
Sally Nightingale had earlier said the defence secretary's request had given her "real hope" that her husband may be home for Christmas.
But after Mr Grieve's announcement, she said: "I was extremely hopeful of an early decision on Danny's fate after the announcement of the defence secretary, who asked for a review of his case, so I am very disappointed that the attorney general has refused to do so.
"The appeal is being lodged tomorrow and I hope that he will at least consider reviewing any decision by the Service Prosecuting Authority to oppose the appeal and seek a re-trial if Danny's conviction is quashed."
Sgt Nightingale's lawyers are expected to lodge an appeal against his conviction and sentence on Wednesday, as well as apply for bail. They said they remained confident at the prospect of an appeal.
The father-of-two, who served in Iraq and Afghanistan, was handed an 18-month sentence in military detention after admitting possessing a prohibited firearm and ammunition.
The case sparked widespread outrage from SAS veterans as well as MPs who were due to debate the case on Tuesday evening.
Defence secretary Philip Hammond wrote to Mr Grieve today, asking him to review whether proper consideration was given to the question of Sgt Nightingale's prosecution was in the public interest before the case was brought to court martial.
But a spokesman for the attorney general's office said: "It would be inappropriate for the attorney general to review either the decision to prosecute or comment on the appropriateness of the sentence.
"That is a matter for the Court Martial Appeal Court in due course."
Although the Ministry of Defence (MoD) is unable to intervene directly in the legal case, it is understood that Mr Hammond's personal view is that it is in the public interest for any appeal to be heard as a matter of urgency.
David Cameron was said to be sympathetic for Sgt Nightingale and his family but his official spokesman said: "This is a case where due process has to be followed."
The spokesman told a regular media briefing in Westminster: "It is for the attorney general to rule on these issues. On these issues, the attorney general has a specific role which is separate from that of a Cabinet minister.
"There are due processes to be followed in these matters and it is not for the government to intervene in the findings of a court, whether that is a military court or a civil court.
"It was open to the defence secretary to refer this matter to the attorney general. He chose to do so and the attorney general has taken a judgment. That's the way the process works."
Asked if the PM had any sympathy for the Nightingale family, he said: "Yes, but clearly this is a case where due process has to be followed, where we can't intervene to change a court decision.
"Indeed, this is something that may well in due course be considered by the Court of Appeal."