Celtic fans chanted an anti-Royal Family song during tributes to the Queen before their Scottish Premiership clash against St Mirren.
Hoops supporters sang ‘if you hate the Royal Family clap your hands’ before their match on Sunday.
They also unveiled a banner that echoed the same sentiments.
St Mirren had announced before the match that they would be holding a minute’s applause, rather than silence, in tribute to Her Majesty Queen Elizabeth II.
That decision was made after Celtic had held up a ‘F*** the Crown’ banner before their Champions League clash with Shakhtar Donetsk.
UEFA subsequently launched an investigation and said: “Please be informed that proceedings have been opened against Celtic regarding a banner displayed during their UEFA Champions League match against FC Shakhtar Donetsk.
“The UEFA disciplinary bodies will decide on the matter in due course.”
In the run up to the match, Celtic manager Ange Postecoglou said: “I have the same message to our fans that we always have. As I said before the game [against Shakhtar], we will abide by the protocols. We wore black armbands on Wednesday night.”
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He added: “I think there’s a minute’s applause [on Sunday], we will abide by whatever obligations and responsibilities we have as a football club. We will do that in a respectful manner. We want our supporters to do the same.”
Celtic went on to lose their with St Mirren 2-0, which was their first Scottish Premiership defeat in a year.