Chelsea have pledged to ban any supporters found to have used anti-Semitic language during Monday's 4-1 defeat at Watford.
A report on a Jewish website said that one Blues fan was left "disgusted" by chanting from fellow supporters, reporting the incident to stewards at the ground.
Chelsea subsequently issued a statement on Tuesday evening and have launched an investigation in co-operation with Watford and the police.
"Antisemitism has no place in our club, football or wider society," a Chelsea spokesman is quoted by Sky Sports News as saying.
"Anyone found guilty of antisemitic language or behaviour will face action from the club, including bans, and will be asked to attend equality education courses.
"We welcome the fact that Chelsea fans have reported this behaviour, which shames our club.
"We ask all fans who share our vision for a game open to all, to report any kind of discrimination they witness or experience.
"We will be working closely with the police and Watford to identify those responsible and will take the appropriate action."
Chelsea launched a 'Say No to Anti-Semitism' campaign last week as part of their own inclusion work through the Building Bridges initiative.
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