Video assistant referees (VAR) will not be used in next season's Champions League, with the head of Uefa saying he sees "a lot of confusion".
The International Football Association Board, which rules on the game's laws, meets on Saturday to decide whether to approve the technology permanently.
If it does, then Fifa is committed to using the system at the World Cup.
But Uefa president Aleksander Ceferin said: "Fans see the VAR screen all the time but nobody knows how it works."
He added: "We will not use it in the Champions League next season.
"For me, it might be a good project but we shouldn't rush it."
An Ifab report last month said that the VAR system has been accurate in 98.9% of decisions so far during its two-year worldwide trial.