French club Grenoble admitted a misconduct charge of fielding an ineligible player against London Welsh in a Pool Five encounter last Friday.
An independent disciplinary committee deducted Grenoble four match points and fined them 10,000 euros (£8,120) for fielding Lotu Taukeiaho in the fixture, European Rugby Cup announced.
Taukeiaho was not registered with ERC as a member of Grenoble's tournament squad, and was therefore an ineligible player.
Grenoble won the game 20-9, but London Welsh have now been awarded a 28-0 victory and five match points.
It means the Aviva Premiership newcomers move from third to second in the pool, now three points above Grenoble, and five behind group leaders Stade Francais.
Only the group winners secure a quarter-final spot, though, and London Welsh still have to tackle Stade in Paris.
The misconduct complaint against Grenoble was brought by ERC.
"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," ERC said, in a statement.
"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.
"Both Grenoble and ERC have the right to appeal the decision."
London Welsh and Grenoble meet in the return fixture at Oxford's Kassam Stadium on Saturday.
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