Legia - expelled from the competition after fielding an ineligible player in their 6-1 aggregate victory over Celtic in the third qualifying round - saw their appeal to UEFA dismissed on Thursday.
A UEFA statement read: "The UEFA appeals body met yesterday following an appeal by Legia Warszawa against the decision taken by the UEFA control, ethics and disciplinary body on 8 August.
"The appeal lodged by the Polish club was rejected and, therefore, the original decision of the UEFA control, ethics and disciplinary body is confirmed.
"The control, ethics and disciplinary body had sanctioned Legia for fielding a suspended player (Article 18 of the Regulations of the UEFA Champions League, 2014/15 competition, and Article 21 of the UEFA disciplinary regulations, 2014 edition) in the 2014/15 UEFA Champions League third qualifying round return leg against Celtic FC in Edinburgh on 6 August.
"That match has been declared as a forfeit, meaning Legia lost 3-0."
That does not signal the end of the matter, though, with the Polish champions set to take the issue to CAS in a bid to play Maribor in the play-off round, with a spot in the group stages of Europe's premier club competition up for grabs.
Legia's co-owner Dariusz Mioduski told Sky Sports: "We felt that we had a very good chance of actually having UEFA overturn its own decision.
"We believe that this is not just about Legia, that this is about football generally, and a sense of fairness and justice.
"Of course we will have to adopt a fast-track method. We will have to work today and tomorrow [to] lodge the appeal [with CAS]."
Legia believed defender Bartosz Bereszynski had previously served a three-match ban when they introduced him in the 86th minute of their 2-0 second leg win, but UEFA disagreed, stating that Beresynski was not registered for the club's second-round tie with St Patrick's and his suspension was therefore not fulfilled.
The governing body subsequently awarded Celtic a 3-0 win, enough to see them progress to the play-off round on away goals.
Legia sent an open letter - signed by Mioduski - to Celtic after UEFA ruled they had broken competition rules, asking to play the Scottish club in either Warsaw or Glasgow to conclude matters.
Celtic were unwilling to comply, though, and will now feel confident of their spot in the play-off round despite another Legia appeal.
Celtic defender Charlie Mulgrew said on Thursday: "To get another chance is a great feeling.
"There's disappointment about how the two games went, but rules are rules. We're happy."
The Scottish champions are set to play Maribor over two legs, with the first taking place in Slovenia next Wednesday.
Despite Mioduski's comments, CAS secretary general Matthieu Reeb confirmed to Sky Sports that Legia were yet to file a formal appeal on Thursday afternoon (local time).
