The Scottish Premiership side were originally eliminated from the competition at the qualifying stages by the Polish outfit over two legs, losing 6-1 on aggregate
But now Ronny Deila has been handed a lifeline after a clerical error, and Celtic will enter the pot for the play-off round - with their opponents already issuing a statement on their official website stating that they plan to appeal the ruling.
The Polish champions had fielded full-back Bartosz Bereszynski for the final two minutes as they ran out 2-0 winners at Murrayfield on Wednesday.
However the 22-year-old had been sent of in Legia's match with Apollon Limassol last season and should have been serving a three-match ban.
Legia admitted soon after their victory over Celtic that they were under investigation by Uefa and the footballing body have now awarded the match to Celtic 3-0, meaning the Deila's men qualify on away goals having drawn 4-4 on aggregate.
The statement read: "The UEFA Control, Ethics and Disciplinary Body has met today and announced the following disciplinary decision following the 2014/15 UEFA Champions League third qualifying round tie in Edinburgh between Celtic FC and Legia Warszawa on 6 August 2014.
Legia have been sanctioned for fielding an ineligible player (article 18 of the UEFA Champions League regulations and article 21 of the Disciplinary Regulations). The match has been declared as forfeit meaning Legia Warszawa have lost the match 3-0.
As a consequence, Celtic have qualified for the UEFA Champions League play-offs on away goals (agg: 4-4) and Legia will compete in the UEFA Europa League play-offs.
In addition the player Bartosz Bereszynski has been suspended for one additional UEFA competition match for which he would be otherwise eligible. This suspension shall be added to the remaining two match suspension which the player still has to serve in accordance with the Control and Disciplinary Body decision of 13 February 2014."
It isn't the first time that Celtic have been the benificiaries of Uefa's disciplinary panel, with the Glasgow outfit awarded a Europa League tie against Sion back in 2011 when the Swiss side were penalised for fielding inelgible players.