Legia were stripped of their place despite beating Scottish champions Celtic 6-1 on aggregate because they brought on a suspended player, Bartosz Bereszynski, for the last few minutes of the second leg they won 2-0 at Murrayfield.
UEFA, European soccer's governing body, awarded Celtic a 3-0 win as punishment meaning the Scottish side advanced on away goals after the tie finished 4-4 on aggregate.
In a statement the Polish club said UEFA could have reduced the sanction against them or even "nullify it altogether" if it looked at both the objective and subjectives aspects of their case.
"We wish to inform that the abovementioned argumentation will be used by the club at every stage of the appeal process," Legia said.
Celtic have been drawn against NK Maribor of Slovenia in the Champions League playoff round on Aug. 19 or 20, while Legia are due to play in the Europa League against Aktobe of Kazakhstan.
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