London Irish have invited Heineken Cup organisers to investigate an alleged incident of racial abuse after last Friday's match against Ulster.
Exiles wing Delon Armitage claims he was abused by an opposition player, whose identity has not been revealed.
In a statement, the club said: "London Irish has forwarded its version of events to the tournament organisers and awaits their response."
Ulster have said they are "comfortable" that the allegation is unfounded.
But since the match, London Irish claim to have received a formal apology from an Ulster player for what they describe as "foul and abusive language".
London Irish director of rugby Brian Smith said that he had already lodged a verbal complaint to ERC chief Derek McGrath, who attended Friday's game.
He told the BBC Berkshire that there was "no love lost" between the sides after two recent ill-tempered games.
Smith also claimed that playmaker Mike Catt was "taken out by Neil Best" in the first minute of the game - the England international went off injured at half-time.
"As a result, he (Catt) carried a shoulder injury throughout the first half. Neil Best got a penalty and we might be without Catty for up to a month," said Smith.
Ulster chief executive Mike Reid has said his club have "an unblemished record in relation to racial issues".
"We would take this issue most seriously if the allegation is formally made," he added.
"But from my initial discussions with players, I am comfortable that the allegation is unfounded."