Police raided the headquarters of the SDS and the party's offices at the National Assembly, looking for evidence of abuse of office connected to Šircelj's stint in 2013 as the chairman of the board of directors at BAMC.

Šircelj was appointed to the BAMC board as a state secretary at the Finance Ministry by the SDS-led government in early 2013. He was dismissed in September that year under a law that prevents MPs from holding two jobs.

The STA has obtained a search warrant which shows that Šircelj is suspected of aiding in abuse of office.

The warrant involves the seizure of documents, electronic correspondence and other evidence relevant to decisions to appoint Torbjörn Mansson as BAMC acting executive director and to hire Quartz+co, of which Mansson was a partner, to provide consultancy support.

Among other things, the investigators are looking for evidence on how non-executive directors got in touch with Mansson, proofs of Šircelj's trips to Stockholm, evidence of why Mansson paid for Šircelj's hotel room in Stockholm, evidence of informal agreements on Mansson's work and pay, and minutes of meetings.

Searching BAMC headquarters in October, the police had said the probe targeted abuse of office related to the payment of invoiced consultancy services in the total amount of EUR 2.8m.

The investigation was based on a report from the Court of Audit, which issued an adverse opinion on the BAMC's operation in 2013, suspecting crime in relation to contracts with external consultants.

The investigation prompted SDS deputies to walk out of the ongoing session of the National Assembly, and gave immediate rise to allegations that it was politically motivated.

SDS leader Janez Janša described it as an excuse to seize the main server at the party headquarters.

"It [the server] hosts electronic mail of between 15,000 and 20,000 our members. It's a bizarre excuse to open proceedings against one member and thus get at entire electronic communication of the biggest opposition party."

Police later clarified they were not after the server but merely sought the electronic correspondence of Šircelj's. The server was not taken.

Janša later described it as am manoeuvre designed to distract attention from Sunday's referendum on marriage equality.

SDS deputy group leader Jože Tanko added that "several coincidences hint this is a political project of the [ruling] Modern Centre Party". Šircelj himself said it was a "discreditation attempt".

But the allegation of political interference quickly turned against the SDS, after SDS deputy Branko Grims as the head of the parliamentary Intelligence Oversight Commission sent a delegation to the National Bureau of Investigation (NBI).

The delegation comprised only deputies of the SDS and the ruling Modern Centre Party (SMC). A record of their check will be debated by the commission behind closed doors tomorrow.

United Left (ZL) deputy Franc Trček, who is a member of the commission but was not formally notified of the planned field trip, condemned the visit to the NBI as an attempt to meddle in the investigation.

Police also protested against the move, the commission did not have jurisdiction to carry out the specific check, as the investigation did not involve covert measures, which parliament can supervise.

"We realise how important it is that parliament keeps a check on the work of security services in the execution of covert measures, but we will not allow such pressure to affect the investigation," the police said in a statement.

The case also reignited debate about police access to the premises of the National Assembly, which first came into focus in March, when Alenka Bratušek, an MP and former prime minister, was investigated over suspected abuse of office.

Bojan Dobovšek, an independent MP and security expert, said the police raid constituted encroachment by the executive branch into the sphere of the legislative branch of power.

Matjaž Han, a deputy of the coalition Social Democrats (SD), suggested a special protocol should be adopted to cover cases where police need to enter parliament.