London - Leeds United have apologised to English second-tier rivals Derby County after manager Marcelo Bielsa admitted sending a spy to the Rams' training ground ahead of Friday's match.
Leeds extended their lead at the top of the Championship with a 2-0 win over Derby on Friday.
Derby released a statement before kick-off saying police had been called to their training complex on Thursday to investigate an intruder "acting suspiciously," in an incident that left Rams boss Frank Lampard furious.
Leeds said Saturday they would remind Bielsa and his backroom team of the "integrity and honesty" expected at Elland Road.
"Following comments made by Marcelo Bielsa yesterday the club will look to work with our head coach and his staff to remind them of the integrity and honesty which are the foundations that Leeds United is built on," a club statement said.
"Our owner Andrea Radrizzani has met with Derby County's owner Mel Morris to formally apologise for Marcelo's actions. We will make no further comment on this matter," the statement added.
Bielsa, formerly in charge of Argentina, Chile and a host of European clubs, said Friday he had used the practice of spying on opposition training sessions for nearly 20 years dating back to Argentina's 2002 World Cup qualifying campaign.
"It's true there was someone from Leeds United," the Argentinian told Sky Sports. "The responsibility for this lies with me. I'm responsible.
"It doesn't matter if it's legal or illegal, or right or wrong. For me it's enough that Frank Lampard and Derby County feel it was not the right thing to do and that I didn't behave well.
"I didn't ask permission from Leeds United to do it so it's my responsibility.
"Without trying to find a justification, I've been using this kind of practice since the qualifications for the World Cup with Argentina."
England's governing Football Association has now launched an investigation and Lampard believes Leeds could face a sanction if Bielsa has been spying on the opposition all season.
"He's admitted it so it's easily dealt with. It's up to the league to see how they deal with it," Lampard said.
On the field, Leeds snapped a three-game losing streak with goals from Kemar Roofe and Jack Harrison opening up a five-point lead over Norwich and a seven-point cushion in the race for automatic promotion.
Lampard and Bielsa shook hands before the match, but the Derby manager said his counterpart was guilty of unsportsmanlike conduct.
"At least, on a sportsman's level, it's bad in my opinion," the former Chelsea and England midfielder added.
"If we're going to start talking about 'culturally, I did it somewhere else' -- that doesn't work for me.
"If I'm lucky enough to do well and travel to another country I'll find out what the etiquette is in that country and abide by that."