Rotterdam-based journalist Micha Kat publishes news site klokkenluider.nl, a site that claims to expose corporate corruption.
Responding to a story about Arie Gerla, former head of the Dutch association of shareholders, four anonymously posted messages accused Mr Gerla of falsifying documents and defrauding the City of Rotterdam. Mr Kat had refused to remove the messages.
The council, an independent body that helps to self-regulate the profession, has no legal authority to penalise Mr Kat. But the council stated that comments posted online are comparable with readers' letters in a print newspaper and should be subject to the same scrutiny.
It also said that refusing to remove an offending message reflects on a journalist's professional conduct.
Responding on his website, Mr Kat said: "This site clearly states that comments remain the property of the poster and we are not responsible for their content.
"Clearly the council has not taken account of this."
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