London Welsh will go into next Sunday's Aviva Premiership game against Saracens knowing they may no longer be in control of their top-flight survival.
The Premiership newcomers who play at the Kassam Stadium, Oxford, have been summoned to appear at a Rugby Football Union hearing next Tuesday.
They are charged with fielding an ineligible player in "a number" of Premiership games this season.
The issue relates to the registration of scrum-half Tyson Keats, who, Press Asociation Sport understands, was ineligible for nine of the 13 Premiership fixtures he featured in.
Welsh are currently three points above bottom club Sale Sharks and two behind 10th-placed London Irish.
Should the Keats matter go against them, they could be facing a considerable points deduction - possibly enough to relegate them - and probable fine.