The Football Association has launched a formal investigation into allegations Mark Clattenburg used "inappropriate language" towards John Obi Mikel and Juan Mata during Chelsea's acrimonious defeat to Manchester United.
Clattenburg was accused by the European champions of making comments which are understood to have been interpreted as racist during Sunday's Barclays Premier League game at Stamford Bridge.
Chelsea made an official complaint to the match delegate, whose report appeared to have been passed to the FA on Monday.
A statement from English football's governing body read: "The FA has begun an investigation relating to allegations made following Sunday's fixture at Stamford Bridge between Chelsea and Manchester United (Sunday 28 October 2012).
"The FA will make no further comment at this time."