Rugby chiefs live in perpetual fear of another serious injury overshadowing the World Cup and the tournament format may have to be changed to rest players, World Rugby president Bernard Lapasset said. Worrying about injury "is permanent because the impacts are extremely strong", Lapasset said ahead of the start of the World Cup on Friday. "The physical mass of players has grown. Rugby has become a sport of confrontation and not a game where moving the ball is the priority. The physical mass of players has grown. Rugby has become a sport of confrontation and not a game where moving the ball is the priority Bernard Lapasset "It is a danger that is a risk to the players," said Lapasset, who spoke a day after former Wales international Leigh Thomas retired saying that repeat head blows had caused his epilepsy. The most serious injury yet in a World Cup came in 1995 in South Africa. Ivory Coast's Max Brito was left paralysed below the neck after being trampled in a ruck in a pool game against Tonga. Lapasset said "intensive" medical preparations were ready for the tournament, which starts with England taking on Fiji at Twickenham. Hawk-Eye cameras used to help referees will also monitor players to see who are at risk after any blow to the head. "As soon as there is a potential risk after a blow or a tackle, we act very quickly to accelerate the treatment process," said the 67-year-old Frenchman. But the global body's chief acknowledged rugby's image problem. "The health of the player is the priority of priorities. You cannot have a sport that leaves players injured, that leaves them concussed," he said. "We have to be a sport that everyone can play." The growing demands on rugby players could also force a change to the tournament but no process has been started yet, Lapasset said. "The key element is the rest time for players. Maybe we will be forced to change the format of the competition. Maybe it needs to be a two stage competition. I don't know." Lapasset said the World Cup needs a large number of teams to play top-quality rugby but to achieve that goal top-level players need to be spread out over a wider number of countries. Four nations reached the 2015 World Cup through play-offs - Georgia Namibia, Romania and Uruguay - rather than getting direct qualification through the rankings. "That means these teams are not yet at professional maturity," said Lapasset. "Perhaps one day there will be 24 countries in a different format: with a qualification phase and a final phase. "It is a decision to think about so that the World Cup remains a major event." Lapasset said, though, that the World Cup in England would be an enormous success. "It is a chance for England to show the strength that rugby represents at a national level. You can feel there is an extremely strong fervour. Perhaps one day there will be 24 countries in a different format: with a qualification phase and a final phase Bernard Lapasset "There has been enormous demand for all the matches. We are nearly sold out. For the first time we are going to reach 98 per cent of tickets sold." In May next year World Rugby will hold a presidential election. Lapasset is also co-head of the French committee bidding for the 2024 Olympic Games. The Frenchman, one of the most respected figures in international sport, said he had not yet decided whether to stand again for the rugby post he had held since 2007. He acknowledged that it would be "complicated" during the World Cup and said he would hold talks with federation chiefs during the tournament. Lapasset said he would be "proud" to be part of the French bid for the Olympics, but he could not lead it alone.