Arsenal have launched an investigation after a video appeared on social media appearing to show a fan aiming racist abuse at Napoli defender Kalidou Koulibaly during Thursday night’s Europa League quarter-final.
The incident took place during the first half of the 2-0 win in north London after Koulibaly had cleared a corner that had been swung in by Lucas Torreira.
Arsenal are now working to identify the supporter shown in the 15-second clip and say that anyone caught using racist language will be banned from games and that they are not welcome at the club.
A statement released by the Gunners said: "We utterly condemn the use of any racist language and have launched an investigation to identify the culprit.
"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."
The incident came on the same night that a video emerged appearing to show some Chelsea fans calling Liverpool star Mohamed Salah "a bomber" ahead of their Europa League quarter-final at Slavia Prague.
Chelsea say they will also look to ban their supporters if they can identify them and establish for certain that it happened in Prague ahead of the match, as suspected.
A statement read: "Chelsea FC finds all forms of discriminatory behaviour abhorrent and where there is clear evidence of Chelsea season ticket holders or members involved in such behaviour, we will take the strongest possible action against them.
"Such individuals are an embarrassment to the vast majority of Chelsea supporters who won’t tolerate them in their club."
Liverpool released a statement of their own in relation to the incident.
"This behaviour needs to be called out for what it is - unadulterated bigotry," the statement read.
"As pertains to this latest incident, the club is working with Merseyside Police to ascertain the facts around this footage with the aim of identifying individuals featuring in it.
"In addition, we are working directly with Chelsea Football Club on the matter. We thank them for their condemnation and a commitment to act urgently to identify any individuals responsible."