London Welsh have been charged with fielding an ineligible player in a number of Aviva Premiership matches this season.
The Exiles are to appear at a Rugby Football Union competitions hearing on March 5 over the registration of scrum-half Tyson Keats, and if found guilty they could face a considerable points deduction and probable fine.
It is understood that New Zealander Keats was ineligible for nine Premiership games that he appeared in this season.
London Welsh 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.
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."
The RFU also confirmed that Welsh's former rugby manager Mike Scott is the subject of a separate disciplinary hearing relating to the registration of the same player. His case will be heard at a later date.
Welsh are currently three points above bottom club Sale with six games remaining, but have lost their last six Premiership outings.
The news will be a hammer-blow to the club, who were promoted via last season's Championship play-offs.