Chelsea make complaint against Mark Clattenburg
Chelsea have made a formal complaint against referee Mark Clattenburg after accusing him of using "inappropriate language" to two of their players in the defeat by Manchester United.
Part of the allegation is that Clattenburg used racial language, BBC Sport has learned.
It is also understood midfielder John Mikel Obi is involved in the complaint.
"We have lodged a complaint to the Premier League match delegate," said a Chelsea spokesman.
"The match delegate will pass the complaint to the Football Association."
Referees' organisation Professional Game Match Officials (PGMO) later issued a statement which said: "PGMO is aware of the allegations and they are being treated with the utmost seriousness.
"Mark will co-operate fully and welcomes the opportunity for the facts to be established."