Sion have vowed to fight their Europa League expulsion with an appeal to UEFA and Swiss civil court action after describing the decision as "pure revenge" against controversial president Christian Constantin.
On Tuesday Sion revealed they had launched an appeal to UEFA, although the European governing body have yet to receive it. Constantin has vowed to take his team to Madrid for the Europa League group opener against Atletico next Thursday despite Celtic being reinstated in the competition at their expense.
In a statement, a Sion spokesman said: "FC Sion received 12 pages of confused explanations (in French) for the acceptance of the protest. The decision comes from the three UEFA judges who decided to keep Celtic in the competition. For us, it's a pure revenge against our president."
The statement continued: "FC Sion officially appealed to UEFA on Monday, in order to obtain the cancellation of the protest.
"We remind that Celtic received before its home game an official document from the European federation: it mentioned that players were officially eligible. Sion prepares its travel to Madrid.
"Players (individually) and the club are now bringing legal actions against UEFA at Swiss civil court. Civil court already gave the players the right to play (before the games), that was not respected by the UEFA. This is a serious violation of Swiss laws and may result in penal proceedings."
Protests from Celtic over Sion playing illegible players in the Swiss side's 3-1 aggregate were upheld on Friday, but uncertainty remains over who, if anyone, will face Atletico on September 15.
A UEFA spokesman told Press Association Sport on Tuesday morning they had not yet received anything from Sion, but the Swiss club also announced they had made an appeal in a brief statement on their official website.
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