Tottenham Hotspur said they would take the strongest possible action after several hundred Red Star Belgrade fans defied a UEFA ban to attend the Champions League Group B tie on Tuesday.
Red Star fans were barred from buying tickets for the clash in north London as punishment for racism by some of their fans during a Champions League qualifier against HJK Helsinki.
Yet a boisterous contingent were making themselves heard in the upper-tier behind the goal, although they did not have much to cheer as their team were hammered 5-0.
It is not clear how they obtained the tickets but Tottenham said they would come down hard on any home fans who sold their ticket on to visiting fans.