The Czech Republic’s billionaire prime minister is back in the headlines. His local council found him guilty of breaching conflict of interest rules by owning major media outlets while holding high office.

Andrej Babiš has withstood mass protests demanding his resignation, a criminal fraud inquiry and an ongoing European Commission investigation into his business practices.

According to the Guardian, however, the latest twist is being described by the prime minister’s critics as a breakthrough verdict. Specifically, the Černošice council – a small municipality of just 7,000 people, including an upmarket village just outside Prague where Babiš lives – upheld a complaint by Transparency International (TI) that he is breaking the law through his ownership of the giant multi-industry Agrofert group, which includes two national newspapers and the country’s biggest commercial radio station.

The council is the first Czech institution to declare that Babiš still controls Agrofert, even though he put the conglomerate into a trust to avoid any accusations of wrongdoing. TI argues that Babiš is the trust’s ultimate beneficiary and that Agrofert remains firmly in his grip.

The council’s finding empowers it to impose only a relatively modest maximum penalty of 250,000 Czech crowns. But it could have wider implications.

“It has implications for the European complaint,” said David Ondráčka, director of TI’s Czech office. “The European Commission has said that while it is looking at the matter and making its own assessment, it is also curious as to how the Czech authorities will deal with it. The possibility of the Černošice ruling being transferred to the European complaint really worries Babiš.”

According to local media reports, Babiš responded angrily to the ruling, which he described as “politicised”, and vowed to challenge it. “I will definitely appeal,” he wrote in a text message to Czech journalists. “I do not understand how officials who deal with traffic offences can decide on such a complex legal problem.”