The EU has warned that anti-Semitism has become "disturbingly normalised".
Chelsea say fans who sang an anti-Semitic chant in the Europa League game with MOL Vidi have "shamed the club".
A vocal minority chanted offensive songs about Tottenham fans early in Thursday's 2-2 draw in Hungary.
That came days after four fans were suspended for allegedly racially abusing Raheem Sterling.
"Anti-Semitism and any kind of racial or religious hatred is abhorrent to this club and the overwhelming majority of our fans," a spokesman said.