The populist billionaire is alleged to have worked as an agent of the former Czechoslovakia secret police.
The former Czech Prime Minister Andrej Babiš has lost an appeal against claims that he collaborated with the communist-era secret police.
On Tuesday, a Slovak court dismissed the lawsuit that was filed by Babiš following the allegations.
The former PM had tried to sue Slovakia’s Institute of the Nation’s Memory (UPN), which holds secret police files following the division of Czechoslovakia in 1993.
The institute said that some files contain evidence that Babiš was an agent under the code name “Bures” from 1982. Babis, who is Slovak-born, has denied the claim.
Bratislava’s regional court had originally rejected his lawsuit in 2018, but the country’s Constitutional Court ordered a retrial.
Babiš repeated on Tuesday that he “never cooperated” with the secret police - known as the StB - and vowed to continue defending himself in courts against the Slovak Interior Ministry.
The billionaire populist currently sits in opposition after his centrist ANO movement lost last year’s parliamentary elections in the Czech Republic. He is expected to run for the role of President early next year.
Babiš has also been charged in a fraud case involving European Union subsidies and his agrochemical conglomerate Agrofert.
The 67-year-old is accused of using €2 million in EU money, which was intended for small and medium-sized companies, to fund his large business
The European Commission has reportedly imposed a €3.3 million fine on the Czech Republic for mistakes in the distribution of EU agricultural subsidies related to the case.
Babiš has always denied the allegations and says they are "politically motivated" and "absurd". He is set to go on trial in September.