An intermediary in the 2018 double murder of a Slovak investigative journalist and his fiancee has been sentenced to 15 years in jail after agreeing to a plea bargain.

Zoltan Andrusko was a businessman and pizzeria owner when he joined the plot to carry out the killings, which triggered mass protests and toppled then premier Robert Fico.

The murders of journalist Jan Kuciak and his fiancee Martina Kusnirova also paved the way for the election of liberal anti-corruption activist Zuzana Caputova as Slovak president in March.

"I consider the 15-year sentence to be appropriate," Judge Pamela Zaleska told reporters after the verdict was announced at the Special Criminal Court in the western city of Pezinok.

Kuciak had been investigating Slovak entrepreneur Marian Kocner's business activities when he and his fiancee were gunned down at home near Bratislava in February 2018.

Andrusko served as a go-between in the murder, allegedly hiring the gunmen on the request of his friend Alena Zsuzsova, who was in turn following orders from mastermind Kocner. 

Kocner, Zsuzsova and alleged gunmen Tomas Szabo and Miroslav Marcek -- who are all in custody -- will be tried in January. Andrusko will appear as a witness. 

Slovak businessman Marian Kocner, suspected of ordering the 2018 assassination of investigative journalist Jan Kuciak and his fiancee Martina Kusnirova. Photo: AFP

According to the indictment, Kocner decided "to get rid of Jan Kuciak physically and thus prevent further disclosure of his (Kocner's) activities" after failing to find "any dirt" to discredit the journalist. 

Kocner, who is also under investigation for his role in several cases of suspected fraud, has a reputation for hostility towards journalists.

Investigators have revealed that Kocner exchanged thousands of messages with senior government officials from the ruling Smer-SD party.

Former premier Fico remains the party leader and is widely seen as still pulling the strings.

Support for Smer-SD has dropped to a historic low of around 20 percent, but it is still poised to win the February general election. 

It might have trouble finding coalition partners however, according to analysts.