UEFA has opened disciplinary proceedings against Celtic over banners on display during their Champions League meeting with Shakhtar Donetsk.
A number of banners referencing the recent passing of Queen Elizabeth II could be seen in the Celtic end at Polish Army Stadium in Warsaw, where the sides played out a 1-1 draw on Wednesday.
Celtic now face potential sanctions, with UEFA confirming to Stats Perform on Thursday that disciplinary proceedings are under way.
However, Celtic's rivals Rangers will not face any punishment for defying UEFA's orders by playing the national anthem in tribute to the Queen before their 3-0 loss to Napoli.
A spokesman for the European football governing body said: "The incident is not the subject of any UEFA disciplinary proceedings."