Leeds United manager Marcelo Bielsa admitted that an employee of the club was caught spying at Derby County's training ground and has apologised to Derby boss Frank Lampard for the incident.
"It's true, there was someone from Leeds there. The responsibility for this incident is mine," Bielsa told Sky Sports on Friday, via an interpreter. "It doesn't matter if this is legal or illegal, right or wrong ... for me it's enough that Frank Lampard and Derby County were unhappy, to show that I didn't behave well. Yesterday I talked to Frank and he told me I didn't respect his club. I have a different point of view, but the important thing is what he and Derby think.
"I'm the only one responsible for it, I didn't ask Leeds for permission to do that. I've been using this kind of practice since the World Cup qualifiers with Argentina. This is not unlawful, we've done it publicly, we talk about it in the press, for some people it's the wrong thing to do -- for others it's not."
Championship leaders Leeds beat Derby 2-0 on Friday with the build-up to the match dominated by reports that a person was arrested for cutting a hole in the fence and spying on the final prematch training session for Lampard's side.
Prior to the match, Lampard called the incident "bad"and told Sky Sports that "if we are going to start talking about 'culturally I did it somewhere else and it was fine,' then that doesn't work for me ... It has disrupted our buildup to this game."
In a statement on Friday, the FA said it is "aware of the incident at Derby County's training ground and we will be investigating the matter."
Derbyshire Police issued a statement denying the reports, but did confirm that "a man was found at the perimeter fence of Moor Farm and was spoken to."
However, Derby subsequently released statement insisting that their privacy was breached by a member of Bielsa's staff.
A club statement read: "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."
Later on Friday, the English Football League (EFL) released a statement addressing reports that Derby had lodged a formal complaint which read: "We are aware of the reports in relation to an incident at the training ground of Derby County. It is, of course, up to Derby County as to how they progress this matter but as of yet we have received no complaint or contact from the Club."
Leeds, who were relegated from the Premier League in 2004, go into the game two points clear of second-placed Norwich at the top of the Championship. Derby were in sixth place, eight points behind Leeds, heading into the trip to Elland Road. Leeds won 4-1 when the two sides met at Pride Park last August.
Last month, German club Werder Bremen admitted flying a drone over Hoffenheim's training ground ahead of a Bundesliga match between the two clubs.
Last summer, Sweden confirmed a member of their coaching staff got into a closed South Korea training session and was asked to leave, leading to a squabble between the teams over spying ahead of their group-stage match at the 2018 World Cup. South Korea coach Shin Tae-yong played down the incident and said he mixed up shirt numbers in an attempt to confuse the scout.