European football's governing body has punished the Scottish league leaders after their supporters unveiled a large banner reading "F*** UEFA" and flares were lit in the crowd during the Group I match at the Stadio Friuli on December 15 of last year.
The banner was made in response to Celtic being fined just two days prior to the match in northern Italy for what was described as 'pro-IRA chanting' by some of their supporters in an earlier group game against French club Rennes.
Celtic had since banned for life a fan who they said displayed the banner, but that did not see them avoid a further fine.
A UEFA spokesperson told PA Sport: "Celtic have been fined 25,000 euros by UEFA's control and disciplinary body for breaching Article 11 of the UEFA disciplinary regulations.
"The decision was taken on February 23. It is nothing out of the ordinary that it wasn't announced then. We can have 100 cases at the monthly meetings. Celtic did not appeal."
The Bhoys, who finished third in their group and crashed out of the tournament, were found to be in breach of Article 11 of UEFA's disciplinary code, which prohibits the use of fireworks and "the use of gestures, words, objects or any other means to transmit any message that is not fit for a sports event, in particular if it is of a political, offensive or provocative nature".