Uefa urges Sion to settle dispute at Court for Arbitration
Uefa has asked Sion to settle their legal dispute once and for all at the Court of Arbitration for Sport.
The governing body threw the Swiss club out of the Europa League for fielding ineligible players in the qualifiers and reinstated Celtic.
And Sion have breached football's rules by launching a series of civil court actions against Uefa.
Sion, who had beaten Celtic to qualify, also filed a criminal complaint against Uefa president Michel Platini.
That came after his organisation's independent judicial body ignored a local court's interim ruling to reinstate Sion.
Uefa has now offered Sion a fast-track case at world sport's highest court to end the dispute "in a common spirit".
European football's governing body says its executive committee met on Monday without Platini to study the stand-off and concluded that only CAS has authority to pronounce a verdict.
Celtic have already played twice in Group I, losing away to Atletico Madrid and drawing with Udinese.