Chelsea face the prospect of a partial stadium closure after UEFA announced it is investigating the alleged anti-Semitic chants by Chelsea supporters during last Thursday's Europa League tie with Vidi in Budapest.
Chelsea condemned the singing of a derogatory chant about Tottenham supporters, featuring anti-Semitic language, by a vocal minority of their own supporters as "abhorrent" and vowed to take the "strongest possible action".
Now European football's governing body is investigating and due to report in mid-January.
The minimum penalty if the offence is proven is a partial stadium closure, UEFA regulations state.
Chelsea's next home game in European competition is February's Europa League last-32 clash with Malmo of Sweden.
A statement from UEFA on Tuesday read: "UEFA has today announced that it has commissioned an ethics and disciplinary inspector to conduct an investigation in relation to the alleged racist incidents that occurred at last week's UEFA Europa League group stage match between Vidi FC and Chelsea FC played on December 12 in Hungary.
"Information in respect to the results of this investigation will be made available by the middle of January."
The incident happened just days after four fans were suspended for abusing Raheem Sterling.
Chelsea and the Metropolitan Police opened investigations after alleged racist abuse was directed at Manchester City's Sterling by a section of home supporters during the match at Stamford Bridge on December 8.
Minutes into the Blues' next game – Thursday night's Europa League clash with Vidi at the Groupama Arena – a vocal minority of Chelsea supporters 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 is disgusted at the latest incident.
A Chelsea spokesman said in a statement last Thursday: "Anti-Semitism and any other kind of race-related or religious hatred is abhorrent to this club and the overwhelming majority of our fans. 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 using anti-Semitic or racist words or actions will face the strongest possible action from the club."
Article 14 of UEFA's regulations state that if supporters are found guilty of behaviour that "insults the human dignity of a person or group of persons on whatever grounds, including skin colour, race, religion or ethnic origin... (the) club responsible is punished with a minimum of a partial stadium closure".
If proven, this would be Chelsea's first offence.
A second offence would see them forced to play a match behind closed doors.
Chelsea assistant manager Gianfranco Zola is a hero to supporters, who he believes will now show respect, following his playing spell at Stamford Bridge.
Speaking ahead of Wednesday's Carabao Cup quarter-final with Bournemouth, Zola said: "It's sad. Racism doesn't belong to football, doesn't belong to anything. Obviously sad and disappointed about that.
"I know these supporters very well. They gave me so much and the team so much. They will react in a positive way, support the team in a strong way, but also in a very respectful way."
