The Polish club beat the Scots 6-1 on aggregate in the third qualifying round tie but paid the price after second-leg substitute Bartosz Bereszynski was found to be ineligible.
Celtic were awarded a 3-0 win, meaning they went through on away goals after a 4-4 draw and were drawn against Slovenian side Maribor in the Champions League play-off.
However, according to Zbigniew Boniek, the legendary player who now runs Polish football, Legia would have kept their place with Celtic's consent.
Legia tried to contract the Celtic before the ruling was announced but Boniek said it was impossible.
"Had Celtic officially given up their place, Legia could have stayed in Champions League," Boniek told Weszlo in quotes translated by British-based Polish journalist Robert T. Błaszczak.
"Celtic were unreachable that day. Didn't return the calls or reply to e-mails. They just hid and waited."
Legia lost their place by bringing on Bereszynski in the 86th minute of the second-leg at Murrayfield.
Bereszynski was handed a three-match ban after a red card in the Europa League last season and Legia believed he was free to play after missing the two games against St Patrick's Athletic as well as the first leg against Celtic in Warsaw.
However, the player was not included on Legia's squad list submitted to UEFA for the St Patrick's games and thus his suspension was not deemed to be served.
Legia will now play Aktobe of Kazakhstan in the Europa League play-off unless they can successfully appeal the decision.
- Sports & Recreation
- Zbigniew Boniek