The international body has rendered the ruling to exclude the Swiss club from the competition admissible, but there is still a possibility of further appeal
The Swiss side were denied a place in the competition after fielding ineligible players in the play-off round against Celtic, while under a transfer ban for illegally signing Essam El-Hadary from Al Ahly in 2008.
As a result, Sion were replaced by the Scottish outfit in Group I, prompting the Swiss team to file an appeal.
"The CAS first decided that it had jurisdiction to rule on the request for arbitration filed by Uefa on 26 September 2011," a CAS statement read.
"It then decided to confirm that Sion should not be reintegrated in the Uefa Europa League 2011/2012, confirming indirectly the validity of the decision of the Uefa Appeals Body of 13 September 2011."
The international tribunal has also lifted the provisional measures ordered by the civil court of the Canton of Vaud, which had ruled in October that Sion should be reintegrated into the competition and paid damages by Uefa.
According to CAS, the Swiss civil court stated that Swiss side had 60 days to file a claim, but the club never did.
"The grounds for the award will be communicated to the parties and published by the CAS within the next weeks," the statement continued.
"Thereafter, an appeal will be possible at the Swiss Federal Tribunal (time limit of 30 days from receipt of the grounds for the award)."