Pierluigi Collina believes today's referees must be athletes if they are to officiate to the best of their ability at the highest level.
The 50-year-old is widely regarded as one of the best referees of all time and currently advises UEFA on improving the standard in Europe.
Interviewed in La Gazzetta dello Sport, Collina said: "The referee must be an athlete. It's impossible to do well without this presumption. And so, the physical preparation is fundamental.
"For the first time we have placed all referees on a valuation of body mass.
"That is the first season, then comes the rest - Knowledge of the teams, the players, the tactics, the weather conditions - all the unavoidable things for a good refereeing performance.
"The game must be prepared for in many aspects."