Slovenia's Constitutional Court has put a final stop to the use of IMSI catchers by the law enforcement authorities after imposing a temporary injunction in 2019 on the use of these devices, which mimic mobile phone towers to intercept mobile traffic.

The legal basis for IMSI catchers was created in the controversial amendments to the criminal procedure act that were passed by the National Assembly in March 2019 under the centre-left government of Marjan Šarec.

However, the provision concerned and several others were challenged by the Democratic Party (SDS) and the Left, two parties from the opposite ends of the political spectrum which were both in opposition at the time. They countered the provisions on the grounds of invasion of privacy.

In a partial ruling issued on 5 January, the court agreed with the petitioners that the provisions governing the IMSI catchers breached the right to personal data protection.

It explained that police need a court warrant to use the devices and examine the data collected in this way. Due to the way these devices operate and the depth and breadth of their reach, the investigating judge should be allowed to conduct a subsequent judicial review of the device's use, which the court found impossible in the current legislative framework.

It said a judge cannot conduct effective review of the constitutional and legal aspects of IMSI use as established through the case law of the Constitutional Court and the European Court of Human Rights.

The court holds that several of the act's provisions are inconsistent with the requirement that a right balance must be struck between the gravity of encroachment on information privacy of the person against whom the measure is ordered and of third parties, and the benefits that would emerge as a result.

The court stayed article 150.a, which provides the legal basis for IMSI catchers, in July 2019, arguing that its enforcement could cause damaging consequences that would be hard to repair. It said the use of the devices could provide the basis for further invasive encroachment on human rights by the state.

The Interior Ministry argued at the time that the use of IMSI catchers would make police work more effective and successful. It said the devices were used "successfully and effectively in several EU countries, helping them combat the worst forms of organised and other crime".

The SDS and the Left challenged several other provisions which deal with covert investigative measures and data collection and surveillance in traffic, arguing grave and disproportional invasion of privacy. One of the provisions they are challenging makes it possible to conduct a house search without the person under investigation being present.