Celtic will discover if their Europa League reinstatement stands on Tuesday - two days before their scheduled opener against Atletico Madrid - after Sion submitted an appeal against their expulsion late on Thursday night.
UEFA confirmed an appeal hearing would be heard on September 13, the day before Celtic had planned to fly to Spain.
Sion had revealed on Monday they planned to appeal and UEFA were expected to convene a panel today, but the delay throws up even more uncertainty for the affected clubs and Celtic fans hoping to travel to Spain.
Sion, who were thrown out for fielding ineligible players, will take their case to UEFA's appeals body after the control and disciplinary body ruled against them last week.
The Swiss club, who beat Celtic 3-1 on aggregate in the play-offs, would have one more conventional avenue of appeal if their second UEFA hearing fails.
The Lausanne-based Court of Arbitration for Sport would be able to overturn UEFA's decision if they found in favour of Sion.
But the club's controversial president, Christian Constantin, has vowed to take on the governing body in the civil courts, although their first attempt failed on Thursday.
Sion failed with an attempt to overturn the decision in their own regional court, with the Valais district chamber ruling it had no jurisdiction over Nyon-based UEFA.
However, the club later revealed that "many civil proceedings" were in progress, and could be more successful at the court in Vaud, where UEFA are based.
Constantin last night revealed a civil action on behalf of the six players had also failed but he stated that another case could be brought under different terms.
He told STV: "It's not finished. We might not make it to Madrid but perhaps after that the justice system might ask the competition to stop and allow our re-integration.
"You can start the competition and then a judge might say stop.
"There are four things yet to be seen, two tribunals that still need to give their verdicts for Sion, the re-trial of the players and the decision to call UEFA."
Earlier this week, Sion claimed UEFA's decision had been based on "pure revenge" against their president.
The Swiss side were handed a transfer embargo for breaching rules over the signing of Egypt goalkeeper Essam El-Hadary three years ago but the club and FIFA have different interpretations over the length of the ban.
Sion have already won a civil court ruling over the issue, which prompted their domestic football authorities to allow them to field the affected players.
And the club are adamant UEFA gave them the green light to play them on the back of that decision.
The players include former Hearts defender Jose Goncalves and midfielder Pascal Feindouno, who scored two goals as Sion beat Celtic 3-1 in Switzerland following a goalless first leg.
Celtic played both legs under protest and officially complained to UEFA after their defeat.
Sion were also drawn against Udinese and Rennes in Group I before being removed from the competition.
