Chelsea have lodged a complaint to the Premier League over "inappropriate language" allegedly used by referee Mark Clattenburg towards two of their players in Sunday's 3-2 defeat to Manchester United.
Reports have suggested that Clattenburg aimed a racial slur at Blues midfielder Jon Obi Mikel.
Chelsea were on the receiving end of two contentious decisions from the referee and his assistants at Stamford Bridge as they slumped to their first league loss of the season.
"We have lodged a complaint to the Premier League match delegate with regards to inappropriate language used by the referee and directed at two of our players in two separate incidents in today's match," said Chelsea spokesman Steve Atkins.
A statement from the Professional Game Match Officials has confirmed that Clattenburg "will co-operate fully and welcomes the opportunity for the facts to be established".