Derby released a statement on their official website confirming the events, and Bielsa has admitted that the 'spy' was there on his instruction.
"It's true it was someone from Leeds United," he told Sky Sports.
"I am responsible for it. It doesn't matter if this is legal or illegal, or right or wrong... for me, it is enough that Frank Lampard and Derby felt it was not the right thing to do, for me to believe that I didn't behave well."
The former Argentina manager also revealed that he had already spoken to Derby counterpart Frank Lampard about the situation.
"Yesterday I talked to Frank Lampard and he said I didn't respect the rules of fair play. I have a different point of view on it, but the important thing is what Frank Lampard and Derby think.
"I am the only one responsible for it because I didn't ask for the permission of Leeds to do it.
"Without trying to find a justification, I have been using this kind of practice since the qualifiers for the World Cup with Argentina. It is not illegal, we have been doing it publicly and we talk about it in the press. For some people, it's the wrong thing to do and for other people, it's not the wrong thing to do."
Lampard condemned the tactic in an interview with Sky and claimed that the same thing had occurred before their previous meeting.
"We had somebody the day before our first game against them which we lost 4-1," Lampard said.
"Leeds can beat you 4-1 because they are a fantastic team but they had someone in the bushes that day. The man was asked to leave but it wasn't followed up like it has been this time.
"On a sportsman's level, it's bad. If we are going to talk about culturally and say I did it somewhere else and it's fine, I don't believe that. It's disrupted our build-up to the game. People will say I am making an excuse, but I will speak like this after the game whether we win, lose or draw.
"The training stopped because the police came on the training ground, then it went away. We were training on team tactics, team shape, personnel, how we are going to press, how we will work off the ball, the fact Harry Wilson wasn't training would become evident, so the person who is watching will see all of that."