Slovakia's Supreme Court on Tuesday overturned the acquittals of a businessman and his suspected accomplice for the murder of a journalist in a case that triggered mass protests and toppled the country's previous government.

Well-connected multi-millionaire Marian Kocner and Alena Zsuzsova will now face a new trial for the murder of Jan Kuciak and his fianc?e Martina Kusnirova, who were killed in their home in 2018.

"The court found that the Specialised Criminal Court made several mistakes in the course of the trial, therefore it returns the case to the lower court," judge Peter Paluda said.

The victims, both 27, were gunned down at home gangland-style after Kuciak wrote several stories on graft and the shady dealings by the influential entrepreneur with ties to senior government politicians.

Prosecutors argued that Kocner ordered Kuciak's murder in revenge for articles detailing his various property crimes.

In a statement in court in July last year, Kocner denied murder.

Slovak businessman Marian Kocner must again face trial on suspicion of ordering the 2018 assassination of investigative journalist Jan Kuciak and his fiancee Martina Kusnirova
Slovak businessman Marian Kocner must again face trial on suspicion of ordering the 2018 assassination of investigative journalist Jan Kuciak and his fiancee Martina Kusnirova AFP / VLADIMIR SIMICEK

"I am not a saint, but I am not a murderer either. I'm certainly not a fool who wouldn't realise what a journalist's murder would lead to," he said.

Of the five original suspects, two confessed and have already been sentenced, including the gunman.

Ex-soldier and contract killer Miroslav Marcek received 23 years in April 2020 for killing the couple.

The double murder plunged the country of 5.4 million people into crisis and triggered the largest demonstrations seen since the fall of communism.

Prime minister Robert Fico was forced to resign and replaced by his populist left-wing Smer-SD party deputy Peter Pellegrini.

Parliamentary elections last year were won by the opposition, paving the way for a new centre-right, anti-graft government.