Fifa has warned the Scottish FA that its plan to use video evidence to hand out retrospective yellow cards for diving would contravene the rules.
Gordon Smith's first initiative could be under threat
The world governing body said that room for retrospective action was limited.
"This refers specifically to incidents which have escaped the match officials' attention," Fifa said in a statement.
"According to the Fifa disciplinary code, article 79, the disciplinary decisions taken by the ref on the field of play during a match are final."
SFA chief executive Gordon Smith's first major initiative since taking up his post could now be thwarted by the parent body.
Smith announced on Wednesday the introduction of a pilot scheme where referees would watch DVDs of their games and decide if any acts of simulation warrant cautions.
I am confident that Fifa will see the benefits in supporting an initiative that aims to crack down on cheats in football
SFA chief executive Gordon Smith
He aims to introduce the scheme fully in January after a trial.
Fifa suggests that it would first have to propose a rule change to the International FA Board, but that does not meet until March.
There is no guarantee either that they would get the rule change through what is a notoriously conservative body.
"Fifa takes very seriously the matter of simulation and of any form of cheating in football and actively supports and participates in many initiatives to promote fair play and eliminate cheating from our sport," it said.
"Nevertheless, regarding this specific initiative, only in certain circumstances, according to article 84, can a disciplinary committee apply certain measures.
"Taking this into account, if a referee has seen an incident during a match but determined that it was not a case of simulation, this decision taken by the referee should be considered final."
But Smith hopes to persuade Fifa that his scheme deserves at least a trial.
"I will happily discuss our plan with Fifa to explain the thinking behind it and what we hope to achieve," said the former Brighton and Rangers forward.
"I am confident that they will see the benefits in supporting an initiative that aims to crack down on cheats in football and eradicate a widespread problem.
"This is a serious issue that needs to be confronted and Scottish football could set the standard for many other countries whose game is also suffering.
"It is important to remember that we are trying this in-house to begin with so that any issues such as this can be ironed out by a working party before the system is formally introduced in January."