Championship promotion hopefuls Leeds United have been fined £200,000 by the English Football League for watching opponents train before matches.
A member of Leeds' staff was found acting suspiciously outside Derby's training ground before the fixture between the two sides on 10 January.
Boss Marcelo Bielsa said he had sent a member of staff to watch every team they have played this season train.
The EFL found Leeds breached rules over treating teams with "good faith".
Leeds have also received a formal reprimand from the EFL, which is bringing in a rule to prohibit clubs from viewing their opposition training in the 72 hours preceding a game unless invited to do so.
The EFL said in a statement that Leeds' conduct "fell significantly short of the standards expected" and it "must not be repeated".
EFL chief executive Shaun Harvey added: "The sanctions imposed highlight how actions such as this cannot be condoned and act as a clear deterrent should any club seek to undertake poor conduct in the future.
"We will now look to move on from this incident and commence the discussions about introducing a specific regulation at a meeting with all clubs later this month."
The Football Association has also issued a formal warning to Leeds, Bielsa and a club video analyst.
"The FA will take appropriate action should further evidence of this nature come to light in the future," a spokesman said.
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Leeds admit wrongdoing
In an extraordinary news conference in the week after Leeds' match against Derby, Bielsa said he "observed all the rivals we played against and watched the training sessions of all opponents".
That led to widespread criticism, though there is no specific rule stopping teams from observing opponents training.
During Bielsa's briefing, he showed journalists how much preparation and analysis he and his staff carry out on each opponent before every game.
Leeds said in a statement: "We accept that whilst we have not broken any specific rule, we have fallen short of the standard expected by the EFL.
"We apologise for acting in a way that has been judged culturally unacceptable in the English game and would like to thank Shaun Harvey and the EFL for the manner in which they conducted their investigations.
"Our focus can now return to matters on the field."
'Unethical' Leeds split opinion
Leeds beat Derby 2-0 in their match last month, and Rams boss Frank Lampard called the Whites' conduct in the build-up to the game "unethical".
"I've never heard of going to a training ground on your hands and knees with pliers trying to break into private land to watch," he added after the match.
Swansea City boss Graham Potter said he had "no problem with it" but Ipswich Town manager Paul Lambert said the incident was "not right".
BBC Radio Leeds' Leeds United commentator Adam Pope
It is important to stress that no specific rule has been broken, but rather than of 'acting in good faith towards other clubs'.
This is something Bielsa has done throughout his time in South America and in Europe, and never has he been reproached for it, in fact it has been accepted.
This is not, for me, a man who has set out to cheat or bend the rules, but rather try and work within the rules to use every avenue available for him to leave no stone unturned to try to win a football match.
He has dedicated so much time and effort into studying the Championship and English football that he may have embarrassed chairmen of other clubs that their employees are not doing as much as they could in their quest to improve their fortunes.
However, it is a relief to fans who feared a points deduction as this was such a grey area in terms of sanction.