There were at least 150 Red Star Belgrade fans at the Tottenham Hotspur Stadium on Tuesday night despite their UEFA ban for racist behaviour earlier in the competition.
The fans were present at Spurs' ground as part of corporate packages and therefore the suspensions did not extend to them as they exploited a loophole in the system.
The Serbians have not allowed to sell tickets to their fans as a punishment for racist chanting, during a July qualifying fixture against HJK Helsinki in Finland.
Spurs were therefore unable to put tickets on general sale for fear of them being bought by Red Star supporters.
Their 48,000 season ticket holders were instead allowed to buy up to three additional seats in an attempt to boost attendance.
The Red Star supporters were caught by home fans, who shared videos on social media.
Daily Hotspur tweeted: 'Not sure they support Tottenham', alongside a video of the fans.
Spurs fan Luke Bailey added that 'quite a large contingent' had made their way into one of the home ends.
Red Star were also fined £44,500 for the charge of racist behaviour.
It is the second consecutive year the punishment has been handed out to Red Star.
Last year their fans were unable to buy tickets for group games with Paris Saint-Germain and Liverpool.
