"The EFL has written to Southampton FC requesting their observations following a complaint from Middlesbrough FC relating to alleged unauthorised filming ahead of the two clubs' meeting in Saturday's Championship play-off semi-final first leg," an EFL spokesperson told Southampton daily newspaper The Southern Daily Echo.
"The alleged incident is said to have taken place on Middlesbrough’s private property by an individual identified to be associated with Southampton.
"The league is treating this matter as potential misconduct under EFL Regulations and will be making no further comment at this time."
BBC Sport reported that a Southampton performance analyst was caught recording Middlesbrough training and photographing tactics on Thursday morning, two days before Saturday's first leg at the Riverside Stadium. The analyst was told to delete the footage and leave the area.
League rules bar clubs from observing, or attempting to observe, another club's training session in the 72 hours before a match.
Southampton could not immediately be reached for comment.