The FA is to investigate Derby's claims that their training session on Thursday was watched by a suspected 'spy' from Sky Bet Championship rivals Leeds.
Derby say a Leeds employee had to be removed by police from the perimeter of their training ground ahead of Friday night's Championship match between the clubs, which is live on Sky Sports Football.
Derby manager Frank Lampard suspended training and took the players off the pitch, after it was discovered that someone was watching the session from outside the fence.
Ahead of Friday's match, Leeds manager Marcelo Bielsa admitted that he did send someone to watch the training session, while Lampard claimed the same thing had happened before the sides met earlier in the season.
An FA spokesperson said: "We are aware of the incident at Derby County's training ground and we will be investigating the matter."
Derby's Friday statement read: "Derby County Football Club can confirm that officers from the Derbyshire Constabulary were called to the Derby County Training Centre at around 11:20am on Thursday 10 January 2019, following reports of a man acting suspiciously outside the premises.
"It has since been confirmed to Derby County that the individual concerned is an employee of the footballing staff at Leeds United Football Club.
"The club is now in discussion with Leeds United club officials in relation to this incident.
"At this time no further comment will be made."
Derbyshire Police said: "Officers conducted routine checks, which were all above board.
"No damage to the fence was found, and the man was stopped outside the grounds.
"No arrests were made and the man was sent on his way."