Chelsea have promised to take the "strongest possible action" against any supporters found to have sung an anti-Semitic chant in Budapest, just days after four fans were suspended for abusing Raheem Sterling.
Minutes into Thursday's Europa League clash with Vidi in Budapest's Groupama Arena, which ended 2-2, a vocal minority of Chelsea fans were heard singing a derogatory chant about Tottenham supporters, featuring anti-Semitic language.
Chelsea in January launched a campaign to raise awareness and educate about anti-Semitism in football and the club, including owner Roman Abramovich, who is Jewish, have expressed disgust at the latest incident.
"Anti-Semitism and any other kind of race-related or religious hatred is abhorrent to this club and the overwhelming majority of our fans," said a Chelsea spokesman. "It has no place at Chelsea or in any of our communities.
"We have stated this loud and clear on many occasions from the owner, the board, coaches and players.
"Any individuals that can't summon the brainpower to comprehend this simple message and are found to have shamed the club by used using anti-Semitic or racist words or actions will face the strongest possible action from the club."
Chelsea and police opened investigations after alleged racist abuse was directed at Manchester City's Sterling by a section of home supporters during last week's match at Stamford Bridge.
That overshadowed Chelsea's 2-0 Premier League win over last season's champions City and the latest incident is sure to be investigated by European governing body UEFA, which said it is awaiting the referee's report.
Chelsea topped Group L in the Europa League to qualify for the knockout stages.