Ecuador's Richard Carapaz has "a health issue" and has pulled out of the World Cycling Championships, where he would have been one of the potential contenders in the road race scheduled for next Sunday.
Carapaz was a shock exclusion from the 2024 Paris Olympic road race, where Ecuador instead favoured the now 29-year-old Jhonatan Narvaez, who will also lead this title bid.
But the 33-year-old 2020 Olympic road race champion and 2019 Giro d'Italia winner, Carapaz, has been on form this season, winning the king of the mountains jersey at the Tour de France and coming fourth at the Vuelta a España.
Carapaz said he had made the decision in agreement with his American team EF, believing it was in his interest to rest.
"The most responsible thing to do is to put my health first and opt for a calm recovery rather than put my professional career at risk," Carapaz said in a statement released by the team.
The world championship road races are usually exceptionally tough in length and the September 27 race to Montreal is 273km long.
Double Olympic champion Remco Evenepoel is the race favourite, with Isaac Del Toro of Mexico, Narvaez and France's Paul Seixas also expected to challenge.