Celtic's Europa League campaign may not be over after all, despite the Scottish club's defeat in Switzerland on Thursday night. Their opponents, the Swiss club FC Sion, face being kicked out of the competition for fielding ineligible players – a situation that could mean Neil Lennon's side being reinstated.

Uefa says Celtic have lodged an official protest that Sion breached a one-year Fifa transfer ban, which was originally imposed in 2008 but took effect in January. The Uefa president, Michel Platini, says Sion fielded summer signings "in clear violation of the ban".

Celtic lost 3-1 at Sion on Thursday but the Swiss side had five players signed with the club still under the transfer ban imposed by Fifa after breaching rules over the signing of the Egypt goalkeeper Essam El-Hadary three years ago. Sion have since been found by Fifa, the court of arbitration for sport, and the Swiss supreme court to have breached the transfer ban but are challenging the decision in a local court.

"The rules have not been respected," said Platini. "FC Sion has not respected the rules of the transfer ban – they signed players and then played those players."

The Uefa general secretary, Gianni Infantino, said the case would be dealt with by the organisation's control and disciplinary body, with a hearing expected on Tuesday.

"The civil court ruling does not affect Uefa," he said. "We will look at our rules and the Fifa rules. There is a ruling by Fifa, CAS have ruled, it went to the Swiss supreme court and everything was confirmed but it has been challenged again.

"It is an interpretation question which is complicated – whether a two transfer-window ban means two transfer windows or parts of several transfer windows. Sion will be in the draw but they can be subsequently excluded. The control and disciplinary body will make a decision certainly before the group stage starts."

Platini also called on the Scottish Premier League to take action over their clubs' failure to qualify for Europe. If Celtic do not get a reprieve it would be the first time since European competition began in 1956 that there have been no Scottish clubs in contention beyond August following defeats for Rangers, Celtic and Hearts.

Platini, speaking before the Europa League draw in Monaco, said: "I am very disappointed with Scottish football which has such a great history in European football. They have such great clubs and it is a shame none of them have qualified but these things are decided on the pitch. The Scottish league have to look at this situation very seriously now.