Manchester United have accused supporters who continued to sing an controversial chant about Romelu Lukaku at Southampton of "disrespecting the player's wishes."
The club have asked for CCTV footage of the away end at St Mary's to help identify the fans who sang the song, which has been condemned by anti-discrimination group Kick It Out, during the 1-0 win.
A United spokesman said: "Manchester United has a zero tolerance stance on offensive chanting and behaviour.
"The club and player have been clear in asking for an end to the chant.
"The club has worked with relevant bodies and supporters groups in trying to eradicate any offensive behaviour and will take further action against individuals if this continues.
"The club is in discussion with the police and has asked for CCTV footage from Southampton and will try to identify those who disrespected the player's wishes not to sing the song."
The song, which refers to the size of Lukaku's penis, was heard after the striker had scored the only goal of the game after 20 minutes.
On Thursday, United tweeted a quote for Lukaku that said he hoped supporters could "move on" from the chant.
Sources told ESPN FC there were no complaints made to police about offensive chanting during the 4-1 win over Burton Albion in the Carabao Cup on Wednesday.
However, after the song returned at Southampton, having previously been heard during games against Basel and Everton, United have decided to act. Jose Mourinho, who was sent off in the closing stages, insisted after the game he had not heard it.
He said: "I don't understand what the fans sing. The only music I understand clearly is one from my opponents when they tell Mourinho to go to a place. That's the only one I understand. But the only thing I will say is the fans were amazing, the support amazing from the first to the last minute."