Tottenham have promised to take the "strongest possible action" after Red Star supporters obtained tickets in the home section for the Champions League tie at the Tottenham Hotspur Stadium.
Uefa had banned the Serbian side from selling tickets to their fans as a punishment for racist chanting in a previous fixture, leaving Spurs with the full allocation.
They did allow Red Star to bring 200 supporters with Category 1 tickets, who were seated in hospitality areas - rather than the designated away end.
But Spurs revealed that a group of Red Star fans managed to gain entry with "illegally obtained" tickets in the home supporters' area before they were stopped by police.
Consequently, the club have promised to take action against any Tottenham supporter who sold their ticket.
A club spokesman said: "Red Star supporters illegally obtained individual tickets on Level 5 and made their way around to one section of the stadium in order to congregate as a group.
"Stewards and police contained the group as it was too large a number to eject. We have launched a full investigation in order to understand how tickets have been acquired.
"We shall take the strongest possible action against any individual found to have sold their ticket on."
Those who attended Tuesday's game would have left in a good mood after Harry Kane and Heung-min Son both scored twice to seal a 5-0 victory against the Belgrade outfit.
