Chelsea announced on Wednesday that they have lodged a formal complaint with the Football Association over allegations referee Mark Clattenburg used "inappropriate language" towards John Obi Mikel.
Chelsea allege that Clattenburg abused Nigerian midfielder Mikel during the European champions' stormy 3-2 loss to Manchester United in the Premier League at Stamford Bridge on Sunday.
Reports in the British media have suggested that Chelsea believe the language used by Clattenburg towards Mikel was of a racial nature.
The club's initial complaint claimed Clattenburg had used inappropriate language towards two players, but the allegation regarding the other player -- thought to be Spanish midfielder Juan Mata -- has been dropped.
The FA announced on Monday that they have launched an investigation into the incident, while the Metropolitan Police revealed on Tuesday that they will also conduct a probe.
The game in question saw two Chelsea players sent off by Clattenburg, while United's winning goal was allowed to stand despite being scored from an offside position.
Clattenburg, 37, reportedly denies the allegations and has vowed to co-operate fully with any investigations.