Derby County have made complaint to the English FA after a suspected spy detained by police at the training ground was found to be an employee of Leeds United.
Frank Lampard had his preparations disrupted after a suspicious man was spotted by club staff.
The Derby boss was forced to cancel the session for a time after police were called when a man was spotted lurking suspiciously near the training pitch
Police detained the man - who was found in possession of binoculars, pliers and a change of clothes
Talks are now taking place between the two clubs
A Derby statement on Friday confirmed initial suspicious that the man was spying in an attempt to gather information to pass on to Leeds said:
"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 10th 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."