Saracens could face more punishment after Titi Lamositele played in a crucial European Champions Cup victory over Racing 92 last month a day after his work permit expired.
The European and English champion is already facing relegation from the Premiership at the end of the season for breaching salary-cap regulations.
Sarries will face an independent disciplinary committee on Saturday (AEDT) after European Professional Club Rugby (EPCR) brought a misconduct complaint against them.
Lamositele came off the bench in a 27-24 win over Racing on 20 January (AEDT), but may have been ineligible because of what EPCR described as "an inadvertent administrative error on behalf of the club".
An independent disciplinary committee consisting of Roger Morris, Pamela Woodman and Jean-Noel Couraud will hear the misconduct complaint in London.
The London club faces Leinster in the quarter-finals at Aviva Stadium on 5 April (AEDT) in a repeat of last season's final.