Legia Warsaw boss Henning Berg vowed his side will appeal against their Champions League exclusion after seeing Celtic advance at their expense.
The Polish club won 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.
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.
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.
Berg, speaking to Sky Sports News, confirmed that the Warsaw club would appeal the decision "all the way" as he struggled to come to terms with the punishment.
He said: "For UEFA to make a decision like this, with all the consequences....it's unbelievable.
"We acted in good faith. We made a mistake in administration with the papers for the St Patrick's games.
"We knew he was suspended and he didn't play in those two games or the first game against Celtic.
"He played in the Super Cup and our league games between those matches, he's been registered with us for all this time and we've not tried to hide anything.
"We've been open and it's just a little small technical mistake in the administration, and the consequences for us now is that we are not able to play in the Champions League which was a dream for all our players and the club.
"It's very, very difficult to take and I think it goes against every intention of fair play and fair competition."
Legia will play Aktobe of Kazakhstan in the Europa League play-off which is scant consolation for Berg - who has the sympathy of compatriot and Hoops boss Ronny Deila.
Speaking at the club's Lennoxtown training complex, Deila said: "It is very strange, I have to say that.
"First of all I feel very sorry for Legia, and my friends from Norway there.
"It is tough to think of that and now we are in the Champions League. That is what UEFA said, we haven't been involved in anything.
"I am a football manager and I have to go with it and now we are preparing for Maribor.
"Legia played well against us, they put in good performances but this is nothing to do with Celtic. It is about UEFA.
"It is not my business. It is a club thing and a UEFA thing."