UEFA's emergency panel has dismissed a Swiss civil court ruling that Sion should be reinstated in the Europa League.
UEFA's appeals body had earlier upheld the Swiss club's expulsion, hours after the district court of Vaud, the region in which UEFA's headquarters are based, had ruled in Sion's favour.
The decision confirms Celtic will face Atletico Madrid in the group stage opener on Thursday night.
UEFA revealed the panel, which contains president Michel Platini and four other members of the UEFA executive committee, had confirmed two earlier rulings to expel Sion for fielding ineligible players after discussing the court verdict.
A statement read: "The 'superprovisional' measures were ordered by the Tribunal Cantonal de Vaud without hearing UEFA and solely based on the arguments of FC Sion.
"The court also thought, incorrectly, that the appeals body could not take its decision before the start of the UEFA Europa League group stage on September 15.
"However, the UEFA appeals body met and rendered its decision as announced today, September 13."