Dozens of Palestine flags were displayed during Celtic's 5-2 win over Israeli club Hapoel Beer-Sheva.
Celtic were fined about $20,900 two years ago after a Palestine flag was displayed at a Champions League qualifier against KR Reykjavik of Iceland [Reuters]
Europe's football governing body UEFA have opened disciplinary proceedings against Celtic after a section of their support displayed Palestine flags during Wednesday's Champions League play-off first leg against Hapoel Beer-Sheva of Israel.
UEFA said Scottish club Celtic had been charged for displays of an "illicit banner" and added: "This case will be dealt with by the UEFA Control, Ethics and Disciplinary Body on 22 September."
The UEFA rule in question forbids the use of "gestures, words, objects or any other means to transmit any message that is not fit for a sports event, particularly messages that are of a political, ideological, religious, offensive or provocative nature".
Many Celtic fans have long identified with left-wing causes, among them the Palestinian struggle.
"It's [got] to do with the sense that the Irish Catholics in Scotland have of being underdogs over several generations," Scottish historian Tom Devine told Al Jazeera. "There is a strong sense of history among that community, even though it's now third-, fourth- and fifth-generation Irish."
Celtic were fined about £16,000 ($20,900) two years ago after a Palestine flag was displayed at a Champions League qualifier against KR Reykjavik of Iceland.
The Scottish champions have been punished eight times in five seasons by the European governing body for supporter misconduct.
The second leg of the play-off tie will be played in Israel next Tuesday, August 23.