London Welsh have been fined £8,470 by an independent Disciplinary Committee after a misconduct complaint was brought against the club by ERC.
The fine arrives because Tyson Keats was not registered properly for the competition when he faced Stade Francais on October 13 last year.
The sanction comes on top of the five-point penalty and £15,000 meted out by the Rugby Football Union in March after Keats played ten domestic games.
"London Welsh have been fined 10,000 euros by an independent Disciplinary Committee on foot of a misconduct complaint brought against the club by ERC," read an ERC statement.
"The misconduct complaint was that London Welsh selected Tyson Keats for the Amlin Challenge Cup round one match against Stade Francais Paris on 13 October 2012 when Keats was not properly registered with ERC as a member of the club's tournament squad, and was therefore an ineligible player.
"ERC awaited the conclusion of a Rugby Football Union (RFU) disciplinary hearing involving London Welsh, as well as London Welsh's subsequent appeal against the RFU decision, before beginning its own investigation into Keats' registration for the 2012/13 Amlin Challenge Cup.
"After considering written submissions, the independent Disciplinary Committee determined that London Welsh were guilty of misconduct in that they had breached the Amlin Challenge Cup player registration rules."