Sgt Nightingale, who arrived at court with his wife Sally, had admitted to having a 9mm Glock pistol and 338 rounds of ammunition at a November court martial.
He was sentenced to 18 months military detention, which was reduced to a 12-month suspended sentence on appeal.
He will now face a retrial after the Lord Chief Justice, Lord Judge, Mr Justice Mackay and Mr Justice Sweeney, were told in court that Sgt Nightingale was placed under “improper pressure” to plead guilty.
William Clegg QC, acting for Sgt Nightingale, said Ian Winter QC, who represented the soldier, warned his client he could get five years in prison sentence if he was found guilty, but could perhaps avoid jail with a guilty plea.
Prosecutor David Perry QC said Winter’s advise was above board and nothing unusual.
Lord Judge said: “The question is whether the uninvited indication [of what sentence could be imposed] by the judge and its consequent impact on the defendant created improper pressure and narrowed his freedom of choice.
“We rule that it did and therefore order the conviction to be quashed.”
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