Uefa's disciplinary inspector has appealed against the decision to find Rangers fans not guilty of disciminatory chants.
Rangers fans were in Villarreal in large numbers
Gerhard Kapl, of Austria, who compiled evidence during matches against Villarreal, has challenged last week's ruling by Uefa's disciplinary body.
He had recommended a £25,000 fine and the closure of a stand at Ibrox.
But Uefa stated that it could not tackle sectarianism, which it said was a particularly Scottish social problem.
Kapl has 10 days to detail his reasons for the appeal to European football's governing body, with a hearing likely to be held in May or June.
Disciplinary body decisions involving racist chanting have been challenged twice during the past six months, with one being successful.
Rangers were fined £9,000 after a window on the Villarreal team bus was smashed ahead of the second leg of the Champions League tie in Spain.
But Kapl's charges also related to alleged sectarian and bigoted chanting during both legs, although his appeal is believed to centre on the first leg at Ibrox.
The allegations surrounded the singing of the song, ''Billy Boys'', and the lyrics ''up to our knees in Fenian blood.''
Kapl's report also cited Rangers fans at Ibrox shouting "F... the Pope".
"The evidence at hand shows that discriminatory abuse has been committed by supporters of Rangers FC," concluded the report.
"Rangers FC must be held responsible for the misconduct of their supporters and a strong message must be sent that they deal with this problem seriously."
Rangers chairman David Murray would only say that "our lawyers are standing by to scrutinise the written reasons behind this appeal".
However, despite his club being found not guilty, Murray had earlier responded to Uefa's investigation by strongly condemning the singing of sectarian songs.
And Scotland's FA is set to alter its constitution to comply with a new directive from world football's governing body, Fifa, that demands action against clubs whose fans display bigoted or racist behaviour.
But the Scottish Premier League has played down reports that Rangers have been cited for instances of sectarian singing during Saturday's game against Aberdeen at Ibrox.
The SPL has yet to receive the report from its match delegate, Alan Dick.
An SPL spokesman said: "We have to wait and see what is reported to us.
"If these issues or areas are flagged up by the SPL then there will be discussions with clubs.
"But there is nothing new. Delegates reporting crowd behaviour is part of the remit.
"The match delegate has to report a number of issues ranging from the referee, to issues of commercial, or issues of fair play, that would cover behaviour of players, club officials and also clubs supporters.
"We have asked them to report on these matters since the initiative was introduced."