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 their controversial president Christian Constantin.
Sion revealed yesterday that they had launched an appeal to Uefa, although the European governing body have yet to receive it.
Constantin has threatened to take his team to Madrid for the Europa League group opener against Atletico a week on Thursday despite Celtic being reinstated in the competition at their expense.
Protests from Celtic over Sion fielding ineligible players in the Swiss side's 3-1 aggregate win were upheld on Friday, but uncertainty remains over who, if anyone, will face Atletico on 15 September.
In a statement a Sion spokesman said: "FC Sion yesterday 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. 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 Uefa. This is a serious violation of Swiss laws and may result in penal proceedings."
A Uefa spokesman said yesterday morning that 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 website.
Sion were placed under 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 five affected players, and the club are adamant Uefa gave them the green light to play them.
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. They have the right to challenge the decision in the Lausanne-based Court of Arbitration for Sport should their Uefa appeal fail.
Meanwhile the Celtic manager Neil Lennon is still shocked and confused by a court decision to clear the man accused of attacking him on the touchline last season.
Hearts fan John Wilson was last week cleared of a sectarian assault on the Celtic manager during a Scottish Premier League game at Tynecastle in May. Wilson told the court that he had lunged at Lennon and struck him on the head during the incident but the jury at Edinburgh Sheriff Court said the charge was not proven.
Celtic chief executive Peter Lawwell later said: "Most people will be baffled and find it difficult to comprehend the judgment and verdict," with Lennon maintaining a silence in the aftermath.
He broke that yesterday, though, telling the Daily Record: "To be honest I've still not got my head round it. I needed to take time out and get away from everything because obviously the jury's decision was a shock and confusing.
"It didn't make me consider getting out because I enjoy what I do. I wouldn't criticise the prosecution for pursuing the religious element of the charges. They were just doing their job.
"It was left to the jury to decide and the jury found it not proven. That is the way the justice system is and I just have to accept it."
The Irishman said he was grateful for the support he has received, though, and has promised to put the matter behind him.
He added: "The public reaction since the verdict has been heartening. Eminent people have given their opinions on it and it's nice to have that sort of backing, to know they are on your side.
"I've never had an apology from the guy, but I'm not going to go down that road. I'm hoping that's the end of it now. I just want to get back to talking about football matters and looking forward to the rest of the season."
Celtic have announced that Morten Rasmussen has joined the Turkish club Sivasspor on loan until the end of the year. The Danish striker has joined the club ahead of the start of the domestic season in Turkey.
Rasmussen was signed by Tony Mowbray in January 2010 and scored three goals in 13 appearances before the end of that season, six of them under Neil Lennon.
But the 26-year-old fell out of favour before the start of last season and was loaned to German side Mainz and Danish club Aalborg without playing another game.Reuse content