Tuesday night's Champions League clash between Tottenham Hotspur and Red Star Belgrade was supposed to be watched by home fans only.
But a number of Red Star Belgrade supporters managed to get into Tottenham Hotspur Stadium, despite a UEFA ban.
Uefa banned Red Star's fans from attending Tuesday night's Group B encounter after being found guilty of racism in a qualifying game against HJK Helsinki in Finland in July.
But that did not stop around 200 or so getting into the stadium in N17 and cheering their team on.
Red Star were prohibited from selling tickets to their fans for the encounter, while Tottenham did not put tickets on general sale for fear of them being snapped up by Red Star supporters.
Yet a large group of visiting supporters did manage to enter Tottenham Hotspur Stadium to watch their team.
Spurs have launched a full investigation into how they got in.
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."
The Red Star fans inside the ground were in full voice throughout the match, despite watching their side humbled 5-0 by Spurs, thanks to goals from Harry Kane (2), Son Heung-min (2) and Erik Lamela.
The two teams will meet each other again in Serbia on Wednesday, November 6.
