The football governing body say they will instead await the judgement of the Court of Arbitration for Sport on the issue.
Sion claimed earlier this month that the civil court of the Canton of Vaud ruled that UEFA should immediately reinstate them in the competition and pay the club damages.
UEFA's Executive Committee held an extraordinary meeting on Monday to discuss the case following the court ruling.
A statement from UEFA read: "The Executive Committee of UEFA unanimously stated that it had no statutory authority to reinstate FC Sion as this was the sole prerogative of the disciplinary instances of UEFA, which operate under the Statutes in an independent manner with no influence whatsoever from the UEFA administration or UEFA Executive Committee.
"As the UEFA disciplinary bodies have twice ruled on this case, the UEFA Executive Committee unanimously decided to await the Court of Arbitration for Sport decision on the merits of the case.
"UEFA wishes to state strongly that, of course, it respects the law and that national law itself recognises the competence of the disciplinary structures that are specific to sport.
"This includes the role of the CAS in Lausanne, whose independence has been recognised by the Tribunal Federal of Switzerland.
"In this specific case, UEFA is constitutionally unable to apply to the letter of the super provisional and provisional measures decided by the civil court.
"UEFA noted that the Tribunal Cantonal de Vaud also recognises the role of the CAS as the competent entity able to decide on the merits of such a case, and gave FC Sion 60 days to initiate proceedings to do so.
"In a common spirit of resolving this matter in the interests of the competition as a whole, UEFA has therefore filed a request for a decision on the merits of the case to the CAS on 26 September asking for an accelerated procedure.
"UEFA sincerely hopes that FC Sion will also now make its best efforts to obtain a fast decision in front of the CAS. Provided it does so, CAS is perfectly able to take a decision in time, allowing everyone to respect the sporting and civil laws.
"Should the CAS decide, on the merits, to re-integrate FC Sion then UEFA will of course follow this ruling immediately, as its Statutes allow."
Sion were expelled by UEFA on September 2 for fielding ineligible players in their play-off against Celtic, with their place in the Europa League awarded to the Clydesdale Bank Premier League side.
The Swiss club appealed against the decision but the appeal was rejected on September 13 and Celtic have played in two group ties, against Atletico Madrid and Udinese.
The statement from UEFA continued: "The FIFA and UEFA Statutes, as well as the Laws of the Game, are public and well known to all.
"Specifically, the Regulations of the UEFA Europa League are approved, published and publicly available well in advance of the start of the competition.
"Every club that enters the UEFA Europa League agrees to and signs off on these regulations, and thousands of clubs have accepted and applied these rules - rules that explain and state the various disciplinary instances that can judge and arbitrate issues arising from the matches.
"FC Sion has also accepted these rules, rules that recognise the CAS as the sole and ultimate arbitration entity for disciplinary issues in the UEFA Europa League.
"The CAS is accessible, unlike Swiss civil justice, to all of FC Sion's opponents in the UEFA Europa League. This guarantees that all clubs are considered equal on and off the playing field and receive the same treatment with respect to the rules.
"This principle of equity is paramount to sport and is unanimously supported by the sporting community and all competition participants.
"UEFA has duly noted the demand of FIFA to have its regulations enforced by national associations and now awaits the consequences.
"UEFA insists that judgement on the merits of the case is the best solution for the respect of the integrity of the UEFA Europa League and its participants.
"It is a route open to all, applying impartially the same rules to all, and ultimately giving a victory, obtained on the field in respect of the rules, its true value."