Voynov was on the ice for the team's Tuesday practice, the league said.
"Such activity is in direct contravention of the terms of the suspension levied Oct. 20, which permit Mr. Voynov to use club facilities and work with team personnel but prohibit his participation in any team-related functions or activities," the league said in a press release.
"This was clearly a mistake on our part and we accept full responsibility," the Kings said in a statement. "It is incumbent upon us to be more vigilant in managing this situation to ensure that Slava’s allowable training activities always remain separate from the team."
Voynov was suspended indefinitely after his arrest on domestic violence charges. He pleaded not guilty to a felony count of corporal injury to a spouse Monday.
On Nov. 21, the day after he was charged with one felony domestic violence count, the NHL announced it would provide the Kings with salary cap relief by allowing them to treat Voynov like a player on long-term injured reserve.
- Dan Gartland