Scarlets have been handed a suspended fine of €10,000 after naming the ineligible Jacob Cowley in the matchday squad for their European Champions Cup clash with Racing 92 on January 17.
Cowley was an unused replacement in the 64-14 defeat to the French side despite not being registered for the competition, leading to a misconduct complaint being raised by European Professional Club Rugby (EPCR).
The independent disciplinary committee that imposed the punishment accepted Cowley had been named in error by Scarlets.
However, due to the violation of EPCR rules Scarlets were handed the suspended fine and warned it will become payable should there be any further infringements before May 2018.
Scarlets' defeat in Paris was the fifth of six they suffered in the group stage of the competition, the Welsh province finishing bottom of Pool Three with just two points.
A club statement, in the wake of the sanction, read: "Scarlets fully accept the independent disciplinary committee's decision.
"The administration process that caused the error has been changed to ensure that a similar situation doesn't arise in the future."