Millwall have pledged to ban for life any fans found guilty of racist chanting during Saturday's win over Everton as the FA launched an investigation into a video which has circulated online.
Millwall's dramatic 3-2 FA Cup upset of Everton was marred by crowd trouble outside the ground and a video which appears to show home fans engaging in racist taunts of the visitors.
The FA has said it is aware of the incident and is investigating, and Millwall followed up with a statement of their own promising strong action if the allegations are proved.
"Millwall Football Club is aware of, and extremely disappointed by, a video circulating online displaying alleged racist chanting during Saturday's victory over Everton," the statement said.
"The club will, as is commonplace following such incidents, work with all relevant authorities during investigations into the matter and look to identify individuals involved. Anyone identified and guilty of such abuse will be banned from The Den for life."
Everton encouraged any fans who attended the game and witnessed incidents in or around the stadium to contact them with any information which may help the investigation.
"Our safety teams are continuing to work with the authorities to support ongoing investigations into unacceptable organised disorder and other fan safety-related matters which marred the FA Cup game at Millwall," a statement said.
Anti-discrimination group Kick It Out condemned the alleged chants and called for action to be taken.
"The racist chanting heard from some Millwall supporters during yesterday's match is disgusting and typical of mass discriminatory chants that continue to go unpunished," a statement said.
"We reiterate our message: racial hatred - and all forms of discrimination - have no place in society, especially not in 2019. There is no reason why a footballing environment should be the exception to this rule.
"Kick It Out wants to see the relevant authorities take swift, firm and decisive action against the perpetrators."
Prior to the match at The Den, a man suffered a slash wound to the face during clashes in Southwark, with the Metropolitan Police saying they believed the fight was between Millwall and Everton fans.
Video posted on social media shows police attempting to separate two large groups as punches were thrown.