Leeds United manager Marcelo Bielsa has admitted sending a member of his coaching staff to spy on Derby County before their English Championship match.
Derby, who were talking to Leeds' officials, said local police had spoken to a man acting suspiciously on Thursday outside their training ground.
"It's true there was someone from Leeds United. I am responsible for this incident," Bielsa said before Friday's match at Leeds' Elland Road.
"Yesterday, I talked to (Derby manager) Frank Lampard and he told me that, for him, I didn't respect the fair-play rules.
"I have a different point of view on it but the important thing is what Frank Lampard and Derby County think.
"I am the only (one) responsible for it because I didn't ask for the permission of Leeds United to do that. Without trying to find a justification, I've been using this kind of practice since the qualifications for the World Cup with Argentina. This is not illegal.
"For some people, it's the wrong thing to do and, for other people, it's not the wrong thing to do," said Bielsa, Argentina's coach from 1998-2004.
An unimpressed Lampard downplayed suggestions of an EFL complaint.
"I don't think we need to, because he's admitted it so it's easily dealt with. It's up to the league to see how they deal with it.
"Obviously, it's not just Derby County. We had somebody the day before our first game against them which we lost 4-1. Now Leeds can beat you 4-1, they're a fantastic team, but we had somebody in the bushes that day - twice this season now."
An English Football League spokesman said: "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."
The English FA said: "We are aware of the incident at Derby County's training ground and we will be investigating the matter."
On Friday, Championship leaders Leeds beat Derby 2-0 to end a three-match losing run.
Home fans chanted "All spies aren't we" and "We're Leeds United, we'll spy where we want".