Tottenham have launched a "full investigation" into how a large group of Red Star Belgrade fans got into their Champions League clash on Tuesday night.
Uefa banned Red Star supporters from attending after being found guilty of racist abuse in a qualifying game in Finland in July.
However, over 200 supporters "illegally" got into the top tier of the Tottenham Hotspur Stadium and congregated in the North Stand before making themselves heard by chanting.
Stewards were forced to segregate them from home fans as there were too many to eject.
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."
Uefa has said that it will wait for a report from the delegate at the ground before deciding whether to investigate.
The ban on away fans impacted Tottenham's ability to sell more tickets as they were not allowed to put any on general sale.
Speaking in the pre-match press conference on Monday, Red Star coach Vladan Milojevic said he was upset his side would not have any supporters inside the stadium.
Milojevic insisted he condemns racism, but expressed a desire to see some away fans in attendance.
"This is the 21st century, it is distasteful to talk about this issue," he said. "I absolutely condemn any racist behaviour.
"On the other hand I would love to see our supporters here tomorrow, they really do help us, we feel better when they are around.
"I am a man of sport and I simply don't really talk about this, but if you are asking me if I condemn this sort of behaviour, I absolutely do."