Veteran Italian Michele Scarponi was killed on Saturday after colliding with a van while out training on his bike, his Astana team said, plunging the cycling world into mourning.
"This is a tragedy too big to be written," Astana said in a statement, paying tribute to the 37-year-old, who won the Giro d'Italia in 2011.
"Ciao Michele," Astana wrote on Twitter.
The fatal crash happened on a crossroad near his home in central Italy and he leaves behind a wife and two children, Astana said.
Citing police sources, the Italian daily Il Resto del Carlino said that the driver of the vehicle "did not see" Scarponi.
The cycling star, whose professional career began in 2002, was in the twilight of his career but proved his enduring quality on Monday when he took his first victory in four years in the opening stage of the Tour of the Alps.
He outsprinted Briton Geraint Thomas and Thibaut Pinot of France to claim the early lead of the race, a key warm-up event ahead of next month's Giro d'Italia, the first of the year's Grand Tours.
Scarponi was gearing up for another tilt at the Giro, which starts on May 5 in Sardinia, when the collision occurred in Filottrano, in Ancona province.
He was set to lead the Astana team at the showpiece after injury to Fabio Aru.
"Immense tragedy. There are no words. Rest in peace my friend," Aru tweeted.
Scarponi won the 2011 Giro after Alberto Contador was stripped of the title for doping. The Italian was himself suspended for doping for 18 months between 2007 and 2008.
Contador was among the first to express his shock, tweeting that he was "paralysed and speechless".
He described Scarponi as "a great person and always with a contagious smile".
Astana said Scarponi had gone home after finishing fourth on Friday in the Tour of the Alps and spent the evening with his young family.
"This morning Michele went out on his bike for an early morning training and there the tragedy happened," the team statement said.
"We left a great champion and a special guy, always smiling in every situation, he was a real milestone and a landmark for everyone in the Astana Pro Team," it said.
"The Astana Pro Team clings to the Michele family in this incredibly painful moment of sorrow and mourning."
Other cycling teams expressed their sadness.
"Our thoughts and prayers go to the family, friends and team of Michele Scarponi, who passed away this morning in a traffic incident," Quick-Step Cycling team said on Twitter.
That was echoed by the BMC Racing Team, who tweeted: "Our thoughts are with you all."
Scarponi's death underlines the dangers faced by professional cyclists training on the roads.
In January 2016 six riders from the Giant-Alpecin team, including Frenchman Warren Barguil and John Degenkolb of Germany, were seriously injured after being hit by a car during a training trip to Spain.
Two months later Frenchman Romain Guyot, a young cyclist with the Vendee U team, died after being struck on his bike.