Mark Clattenburg is facing the prospect of a police and Football Association investigation into allegations he used "inappropriate language" towards Chelsea stars John Obi Mikel and Juan Mata.
Clattenburg has been accused by the European champions of making offensive comments, which are understood to have been interpreted as racist, during their acrimonious Barclays Premier League defeat to Manchester United on Sunday.
Chelsea lodged a formal complaint with the match delegate that saw the FA announce an investigation and Clattenburg stood down from officiating for a week. However the police then became involved after the Society of Black Lawyers demanded they look into whether a criminal offence had been committed.
The man behind the mooted black players' breakaway union, Peter Herbert, sent a letter to the Metropolitan Police Service (MPS), which read: "The Society of Black Lawyers wishes to record the incident that took place at Chelsea FC.
"Our information is that racist remarks were directed at John Mikel Obi and at Juan Mata. If so, that is wholly unacceptable in any circumstances.
"Although this matter may be investigated by the FA, it is appropriate that the MPS independently see if a racially-aggravated offence has occurred."
The police confirmed they had received the letter on Monday night but it is understood it was by no means certain an investigation would follow.
It was unclear whether Mikel, Mata or Chelsea would welcome police involvement, though they themselves had made no complaint as of Monday night.
Clattenburg has vowed to co-operate fully with any investigation and has been promised the "full support" of the referees' union.
Anti-racism group Kick It Out refused to comment on the unfolding story until the investigation was complete.