Real Madrid are ready to take their Champions League case to the Court of Arbitration for Sport if UEFA doesn't rule in their favour.
UEFA are still to make a judgement on Real's appeal that both Klaas-Jan Huntelaar and Lassana Diarra be cleared to play in the Champions League, despite the pair having appeared in the UEFA Cup already this season.
Real have submitted a 200-page appeal, but UEFA are taking their time, despite it being lodged last Friday.
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UEFA have rejected Real Madrid's latest efforts to register new signings Klaas Jan Huntelaar and Lassana Diarra for the knockout stages of the Champions League.
In a statement on Thursday, European soccer's governing body said its disciplinary body had upheld UEFA's original decision stating that only one of the players could take part when the competition resumes in February.
UEFA added that any further appeal against the decision would be heard next week.
The Champions League's regulations state that clubs can register up to three new players for the knockout stage by February 1 but only one who has already played this season in Europe with his previous club.
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Netherlands striker Huntelaar and France midfielder Diarra have already played in the UEFA Cup this season for Ajax Amsterdam and Portsmouth respectively.
UEFA's administration had already formally told Real Madrid on Monday that there could be no exception to the regulations.
A UEFA spokesman said earlier on Thursday that Real had however decided to seek a formal review of the case from the disciplinary body.
Real, who face Liverpool in the Champions League's last 16, have told the media that they are prepared to take the matter as far as the Court of Arbitration for Sport if UEFA do not accept their arguments.