Derby manager Frank Lampard has revealed that his team's training sessions have been spied upon by Leeds on two occasions this season.
Leeds manager Marcelo Bielsa accepted responsibility for his club spying on Derby's training session on Thursday, saying that it was his idea.
Police were called to Derby's training ground following reports of a man acting suspiciously.
Lampard revealed that Derby don't intend to lodge a complaint about the matter but added that Leeds also spied on his team's training session before the sides met in August, with Bielsa's team winning that game 4-1.
Speaking to Sky Sports, Lampard said: 'I don't think we need to [make a complaint] because he has admitted it. It's up to the league to see how they deal with it. Obviously it's not just Derby County. We had someone the day before we lost 4-1 against them.
'Now, Leeds can beat you 4-1 they are a fantastic team but we had someone in the bushes that day so twice this season now. The man was asked to leave but it wasn't followed up like it was this time.'
Despite not intending to make a complaint, Lampard expressed his frustration with the situation, which he feels shows a lack of sportsmanship from Leeds.
Asked about Bielsa taking responsibility, Lampard said: 'Well that's good, I spoke to him yesterday afternoon myself and he did the same to me so I'm pleased he's done it again and said that. I think it makes it easier to deal with because at least, on a sportsmans' level, it's bad in my opinion.
'If we are going to talk about culturally I did it somewhere else and it was fine then that doesn't work for me because I don't believe that it is fine on that level. It has disrupted our build-up for this game.
'The fortunate thing is that it has disrupted us without really doing anything. People will stand here and say I'm trying to make excuses before the game, I will speak like this after the game as well, win, lose or draw because of what's happened.'
Explaining the situation, Lampard outlined that the spy will have been privy to important tactical information about his team, including the absence of Harry Wilson.
Lampard said: 'Well the training stopped because the police came on the training ground so that's disruption number one, then it went away. So I actually was made aware of it after training and then later with a phone call.
'What I will say is that we were training the day before the game on team tactics, team shape, personnel, how we are going to press, how we are going to work on the ball, the fact that Harry Wilson wasn't training will become very evident, so the person watching sees all of that.
'If somebody wants to say that isn't relevant and won't affect the game and tactics aren't relevant and how you want to prepare, then that means Pep Guardiola, Mauricio Pochettino, Jurgen Klopp and all the great managers then they are just lucky because if preparation and tactics are not part of the game and aren't sacred that you can work on your own training ground then they are lucky managers because they must be.'
Although the practice of spying on opponents' training sessions may be more common in other countries, Lampard believes that isn't a suitable excuse for Leeds' actions.
He said: 'No. I don't care if it's cultural because if it's cultural and it happens in another country and if I am ever lucky enough to travel and potential possibilities comes up for me later on in my managerial career, when I travel to that country I will find out what the etiquette is in that country and I will abide by it which I think is a good thing actually to do in life not just football.'