Slovakia's constitutional court has ordered a lower court to review claims that the man who hopes to be the Czech Republic's next prime minister collaborated with the Czechoslovak communist-era secret police.
The court ruled at the request of Slovakia's Institute of the Nation's Memory, which holds parts of billionaire Andrej Babis' secret police files.
The institute said they contain evidence Mr Babis was an agent.
Mr Babis denies the claims and Slovak courts previously ruled there was no proof for the allegation.
Mr Babis' Ano movement is a clear favourite to win next week's Czech parliamentary election, paving the way for him to become prime minister.
Today's verdict returns the case to a regional court in Bratislava.
It could be a further complication for Mr Babis, a former finance minister, after he was recently charged by Czech police with fraud linked to EU subsidies.