BRATISLAVA, Slovakia (AP) — Slovakia's Constitutional Court ordered a lower court on Thursday to look again at 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 based in the eastern city of Kosice ruled at the request of Slovakia's Institute of the Nation's Memory, which holds parts of Slovak-born billionaire Andrej Babis' secret-police files.

Some of the files had been destroyed but the institute said 12 of them that survived contain evidence that Babis was an agent.

Babis denies that, and Slovak courts previously ruled that there was no proof for the allegation.

But the Constitutional Court said there are doubts about the trustworthiness of the witnesses on whose testimonies the previous verdicts were based because they were former secret police officials. The court also cited procedural mistakes.

Babis' ANO movement is a clear favorite to win next week's Czech parliamentary election, paving the way for him to become the prime minister.

Thursday's verdict that cannot be appealed returns the case to a regional court in Bratislava. But to clear his name, Babis will have to sue the Slovak Interior Ministry, and not the institute any more, the court ruled.

In his first reaction, Babis told local media he will sue the ministry and "we will win the fight again."

It could be a further complication for Babis, a former finance minister, after he was recently charged by Czech police with fraud linked to EU subsidies.