Premier League referee Mike Dean has been suspended indefinitely over his involvement in a betting website.
Professional Game Match Officials Ltd, which supplies Premiership referees, is investigating whether Dean's links with Arbitros Racing violate his contract.
The website offers shares in a racehorse syndicate as well as advice on how to bet on horses to win or lose.
Dean had been due to referee Sunday's game between Newcastle and Bolton at St James' Park.
The crux of the investigation appears to centre on whether the 36-year-old has played on his status as a well-known figure in football to gain a financial advantage in racing.
He advertised the partnership business in a recent edition of the Racing Post, in which it was stated: "Join Premiership referee, Mike Dean, in this great, new, exciting venture."
Such wording might work against Dean, described as the partnership manager, while the PGMO could also seek comment from Dean as to why he had not informed them of his new venture.
As a business, Arbitros Racing venture would not appear to impact on Dean's footballing duties.
A statement from PGMO said referees must seek approval before they take on additional employment or use their status "for commercial purposes".
The statement read: "The board will investigate whether Mike Dean has breached his employment contract with PGMO.
"The contract requires officials to notify and seek approval of various other activities - including additional employment or the use of their status as a PGMO official for promotional purposes.
"The board has taken this prompt action to protect all parties concerned, and it is right that Mike Dean should not officiate until this matter is resolved to the board's satisfaction."