Janša will not have to return to prison while co-defendants Tone Krkovič and Ivan Črnkovič, who are currently in jail, will be released, according to the court's unanimous verdict released on Thursday.
The Constitutional Court held that the Local Court, as well as subsequently all appellate courts, had failed to present facts that would conclusively show the defendants accepted (Janša and Krkovič) or gave (Črnkovič) the promise of a kickback.
The court reasoned that accepting and giving the promise of a bribe were "independent signs of crime" whose existence cannot simply be inferred from other indications of crime.
Such crimes "must be perceptible in the outside world". If they are not, the rulings must prove the perpetrator's other actions that clearly show that the defendants "expressed the will to give or accept the promise of a reward".
The courts' failure constitutes a violation of the principle of legality enshrined in Article 28 of the Constitution, which states that nobody may be punished for an act which had not been declared a criminal offence under law or for which a penalty had not been prescribed at the time the act was performed.
The court's argument appears to be in line with Janša's long-standing claim that he had been accused of "doing something on an unknown day in an unknown way and in an unknown place".
In Janša's case, the court also found violation of the right to fair trial under Article 23 of the Constitution.
This violation was a result of a Supreme Court judge who was on the panel which ruled on the appeal, having previously criticised Janša's conduct at a public event.
The Constitutional Court held that him being on the panel was inadmissible as the court has to "preserve the image of impartiality in order to affirm public trust in unbiased trials".
The Constitutional Court's decision was unanimous with two concurring opinions. Judge Jan Zobec however was granted a recusal.
The case refers to the 2006 deal with Finnish defence company Patria, from which the Slovenian government wanted to buy 135 armoured personnel carriers (APCs) for 278 million euros. In June 2013, Janša was sentenced to two years in prison and Krkovič and Črnkovič to 22 months each.