London Welsh's Aviva Premiership fate this season could effectively be decided at a disciplinary hearing next Tuesday.
The top-flight newcomers are currently engaged in a relegation scrap with Sale Sharks and London Irish and have only six games left. But they will now face a Rugby Football Union competitions hearing after being charged with fielding an ineligible player, scrum-half Tyson Keats, in "a number" of Premiership games this season.
Press Association Sport understands that the issue relates to nine of the 13 Premiership games in which Keats has featured this season.
English rugby's governing body also said that London Welsh's former rugby manager Mike Scott is the subject of a separate RFU disciplinary hearing relating to the registration of the same player.
Scott has been charged under RFU Rule 5.12 for "conduct prejudicial to the interests of the union or the game". Scott's case will be heard at a later date.
London Welsh have said that they brought the matter to the RFU's attention after conducting an internal investigation earlier this month.
The club also said they wanted to stress than no fault in the matter resides with Keats.
London Welsh chief executive Tony Copsey said: "This is obviously a serious matter which the club has not only brought to the attention of the RFU, but is also working closely with the RFU to provide full co-operation while the case is being prepared and ultimately heard next week.
"Due to the sensitive nature and the impending hearing, the club is unable to make any further comment at this time."
Welsh are currently three points above bottom club Sale. Should the Keats verdict go against them, they could be facing a considerable points deduction - possibly enough to relegate them - and a probable fine.