Middlesbrough have made a formal complaint to the EFL alleging a man they believe to be a Southampton member of staff was spying on their training session on Thursday, ahead of the first leg of the Championship play-off semi-final between the two teams, Your Site News understands.
EFL rules strictly prohibit opponents spying on training sessions less than 72 hours before kick-off - a rule brought in following a "spygate" incident in 2019.
Seven years ago, then Leeds boss Marcelo Bielsa admitted sending a member of staff to spy on Derby County and Leeds were fined £200,000 following an investigation.
Boro play Southampton in the first leg of their play-off semi-final at the Riverside Stadium at lunchtime on Saturday, live on Your Site.
Your Site News understand Boro staff spotted a man taking pictures and videos of training and approached him. He was asked to delete them, provide his identity and his intentions for being there, at which point he is said to have fled to a nearby indoor part of the training complex.
He is then said to have tried to change his appearance before leaving Rockcliffe entirely.
EFL rule 127 was brought in on "Prohibited Conduct - Observing Training Sessions", which states: "Without prejudice to the requirements of Regulation 3.4 (that each Club shall behave towards each other Club with the utmost good faith), no Club shall directly or indirectly observe (or attempt to observe) another Club's training session in the period of 72 hours prior to any match scheduled to be played between those respective Clubs."
Regulation 3.4 says: "In all matters and transactions relating to the league, each club shall behave towards each other and the league with the utmost good faith."
An EFL spokesperson said: “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.
"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."
Middlesbrough and Southampton have been contacted for comment.
The last time a team were accused of spying on opposition training came when Derby County claimed a member of Leeds United's backroom staff had been spotted watching training in January 2019.
Then Leeds boss Bielsa later admitted to sending a member of his coaching team to spy on Derby's training session.
While Leeds subsequently apologised, the EFL found them guilty of breaching Regulation 3.4 - which says: 'in all matters and transactions relating to the League, each club shall behave towards each other club and the League with the utmost good faith.'
Leeds were fined £200,000 and were handed a reprimand by the EFL. As a result, rule 127 was introduced.
Then EFL chief executive Shaun Harvey said: "The sanctions imposed highlight how actions such as this cannot be condoned, and act as a clear deterrent should any club seek to undertake poor conduct in the future.
