UEFA President Aleksander Ceferin has expressed concern over the decision to employ the video replay system (VAR) at the 2018 World Cup, saying the technology still needed more testing before it can be considered for use in the Champions League.
Global soccer body FIFA voted in March to deploy VAR at the June-July tournament in Russia, only two weeks after its use was approved by IFAB, the organisation responsible for rule changes in the sport.
"I have some fear for the World Cup, where we will have referees who have never officiated with the VAR," Ceferin told Italian paper Gazzetta dello Sport in an interview.
"I hope there are no scandals or problems."
Ceferin has already said that VAR would not be used in the Champions League next season.
"The Champions League is like a Ferrari or a Porsche: you cannot drive it right away, you need training, offline testing. And everyone has to understand how it works," he said.
"It's too early for VAR. That doesn't mean we will never have it as the process is inevitable... We'll have it one day in the Champions League but there's no rush."
VAR is already being used in Serie A, the Bundesliga and Portugal's Primeira Liga this season, among others.
IFAB insists it has reduced refereeing mistakes but critics say there has been confusion in a number of matches.
Goals have been annulled several minutes after being scored -- with the teams waiting to restart -- and penalties revoked with the ball on the spot.
Another criticism is that the spectators are not kept informed of what is happening when a decision is reviewed.
Ceferin said that VAR would not have made any difference in last week's Champions League quarter-final between Real Madrid and Juventus, where a soft stoppage-time penalty led to the Spanish side advancing 4-3 on aggregate.
"What would have changed with VAR? Nothing. There are those who have watched it 20 times, I've watched it 50, but for half the people it's a penalty and for the other half, it isn't."