"We used 36 refereeing trios at the last World Cup in Japan and South Korea but that was for 20 venues in two countries," FIFA general secretary Urs Linsi said at a news conference to mark the opening of the workshop.

"The exact number will be decided by the committee but it will not be higher than 36."

Representatives from all six of FIFA's confederations are present in Neu Isenburg on the outskirts of Frankfurt. Linsi said there would be no quota system in place for the finals.

"Performance is the key," he said. "We want to take the best wherever the referees come from."

Referees will be tested on their proficiency in English as well as on their fitness, ability to cope with stress and interpretation of the rules of the game.

"We want the referees to be a team and they should all be able to speak and write a common language," Linsi said.