Grenoble have been deducted four match points and fined €10,000 following a hearing by an independent Disciplinary Committee.
The hearing was as the result of a misconduct complaint brought against the club by ERC.
The misconduct complaint was that Grenoble selected Lotu Taukeiaho for the Amlin Challenge Cup Round 3, Pool 5 match against London Welsh on 7 December 2012 when Taukeiaho was not registered with ERC as a member of the club's tournament squad and was therefore an ineligible player.
The hearing was based on written submissions made by Grenoble, who pleaded guilty to the misconduct charge, and by ERC's Disciplinary Officer, Roger O'Connor.
After considering the submissions, the independent Disciplinary Committee, which was chaired by Roger Morris (Wales) with Rod McKenzie (Scotland) and Jeremy Summers (England), determined that the selection and subsequent participation of Taukeiaho in the match contravened the 2012/13 Amlin Challenge Cup Tournament Rules.
The Disciplinary Committee deducted four match points from Grenoble - ie. the points the club originally gained from the match - and fined the club €10,000 (euros).
London Welsh were awarded a 28-0 win and five match points, and the tournament's Pool 5 table will be amended to reflect the decision.
Both Grenoble and ERC have the right to appeal the decision.