Leeds United manager Marcelo Bielsa astonishingly admitted to spying on Friday night ahead of their game against Derby County.
The Football Association have launched an investigation into the furore that overshadowed Friday's Championship clash.
were first to reveal how Derby boss Frank Lampard was forced to halt training by police on Thursday amid dramatic scenes at the club's HQ, where a mystery man was caught acting suspiciously outside their training ground.
then broke the news on Friday morning that the suspect - and a car in question - was traced back to Leeds United.
Before the clash, Bielsa held his hands up, telling
: 'It's true there was someone from Leeds United. The responsible of this incident is me. It doesn't matter if this is illegal or legal but it is right or wrong.
'For me it's enough that Frank Lampard and Derby County felt it was not the right thing to do and that I didn't behave well.
'I talked to Lampard and he told me that I didn't respect the fair play rules. I have a different point of view on it but the important thing is what Lampard and Derby think.
'I am the only one responsible for it because I didn't ask the club Leeds permission to do that.
'Without trying to find a justification, I have been using this kind of practice since the qualifications for the World Cup with Argentina.
'This is not illegal, we have been doing it publicly, we talk about it in the press, for some people it is the wrong thing to do but for other people it is not the wrong thing to do.'
There were further accusations as it emerged that Leeds may have tried something similar in the reverse fixture earlier this season.
'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,' said Derby boss Lampard.
'The man was asked to leave but it wasn't followed up like it was this time. It's good (that Bielsa has admitted it), 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.'
Earlier in the day, the FA confirmed they have opened an inquiry into the matter after the incident was reported to Wembley chiefs.
An FA spokesperson said: 'We are aware of the incident at Derby County's training ground and we will be investigating the matter.'
It is understood the suspect was in possession of a pair of binoculars and a change of clothes - but police confirmed in a statement he was not arrested over the matter.
Derby held high level talks with Leeds on Friday about the issue, but the Rams have not reported it to the EFL, Lampard confirming that Bielsa's admission would suffice. The EFL, though, could still look to take action against Leeds.
However, there is no spying-related rule in the EFL handbook so it's unclear what punishment, if any, Leeds could face.
However there is a law stipulating clubs must act 'utmost good faith' towards each other - a stipulation Leeds could be in breach of.
In 2014, Crystal Palace were fined after being accused of breaking the rule after it was alleged that they obtained team information before a Premier League game against Cardiff.