Arsenal have condemned racist abuse directed at Napoli defender Kalidou Koulibaly during Thursday night's Europa League match at the Emirates Stadium.
The Premier League club have launched an investigation to identify the individual after social media footage emerged of the incident, which if proven will see them handed a life ban.
Chelsea, meanwhile, denied entry to Thursday night's Europa League win at Slavia Prague for three individuals already identified as being involved in a social media video which included a racially abusive chant about Liverpool forward Mohamed Salah.
Anti-discrimination campaigners Kick It Out praised the swift action of Arsenal and Chelsea, whose security staff handed the trio over to police. The club will seek to ban the six supporters involved in the video.
Recent high-profile incidents, including monkey chants being aimed at England players during a Euro 2020 qualifier away to Montenegro, have again highlighted the problems within the game.
Brighton defender Bernardo has called for those found guilty of racial abuse
Arsenal have urged fans to continue to be vigilant and report any such incidents directly.
"We utterly condemn use of this type of racist language and have launched an investigation to identify the culprit," the club said in statement released to Press Association Sport.
"We operate a zero tolerance approach and anyone behaving like this is not welcome at Arsenal and will be banned from matches.
"We have an extremely diverse community of fans who are all part of the Arsenal family and such incidents are rare at Emirates Stadium.
"We encourage supporters to report any incidents as they happen to stewards or via our matchday alert service."