Rain ruins big names' rainbow jersey hopes
Bronze medalist Fabian Cancellara of Switzerland competes in the Elite Men's Time Trial of the UCI Road World Championships on September 25, 2013 in Florence, Italy
Three Grand Tour champions - Australian Cadel Evans, Britain's Bradley Wiggins and American Chris Horner - were among several riders to abandon prior to the halfway stage of the 272.2 km epic.
Heavy rain began in the early hours of the morning and despite a short respite it began to pour heavily on the 16.9 km course, to be raced 10 times, just as the peloton had arrived in Florence after a 103.2 km trek from Lucca.
Evans, a former world champion and Tour de France winner, was one of several riders to come down near the barriers as the peloton chased down a five-man breakaway in the wet and slippery conditions.
He was later reported to have abandoned by host broadcaster Rai Sport.
The official organisers also announced the retirement of 2012 Tour de France yellow jersey champion Wiggins, part of the British team that was hoping to boost the victory hopes of outside bet Chris Froome, the defending Tour de France champion.
Horner, who won his first Grand Tour at the Tour of Spain earlier this month to come into the race with hopes of a podium place, also abandoned the race.
Belgium's Philippe Gilbert is the defending rainbow jersey champion but faces a stiff challenge from several Italian riders, Slovakian Peter Sagan, Spaniard Alejandro Valverde and Swiss Fabian Cancellara.