Italian cyclist Michele Scarponi has died aged 37 after being involved in a collision with a van during a training ride.
A statement from his Astana team said the crash happened close to Scarponi's home in Filottrano.
Scarponi won the 2011 Giro d'Italia after Alberto Contador was stripped of the title and claimed victory in stage one of the Tour of the Alps on Monday.
"This is a tragedy too big to be written," said the Astana statement.
The statement described Scarponi as a "great champion" and a "special guy", adding: "The Astana Pro Team clings to the Michele family in this incredibly painful moment of sorrow and mourning."
Scarponi leaves behind a wife and two children.
After finishing fourth in the Tour of the Alps behind British winner Geraint Thomas on Friday, Scarponi returned home by car with his masseur before heading out for a ride on Saturday.
Tributes from the peloton
"Devastated to hear the news about Scarponi. Can't believe it. My thoughts with all his friends, family and team," Thomas posted on Twitter.
"Terrible news to wake up to. One of the smiliest, happiest guys in the peleton. Rest in peace," said British cyclist Alex Dowsett.
Spanish rider Contador said: "Paralysed and speechless with the news about Scarponi. Great person and always with a contagious smile. Rest in peace, friend."
Astana team-mate and compatriot Fabio Aru said: "Endless tragedy. There are no words. Rest in peace, my friend."
Specialist climber Scarponi turned professional in 2002 with the Acqua & Sapone-Cantina Tollo team, finishing 18th in his debut Giro d'Italia.
In 2007 he was banned for 18 months after being implicated in Operation Puerto - a major Spanish doping scandal involving some of the world's top cyclists at the time.
Scarponi admitted his involvement in the scandal but denied doping, having been charged with using or attempting to use banned substances and possession of those substances.
Returning in November 2008, he won the Tirreno-Adriatico stage race in 2009 before initially finishing second in the 2011 Giro d'Italia.
He was later awarded his first Grand Tour title after original winner Contador was stripped of his title by the Court of Arbitration for Sport in 2012 after a positive test for clenbuterol at the 2010 Tour de France.
Scarponi was suspended for three months in 2012 by his then team Lampre for visiting doctor Michele Ferrari - who is banned for life by the US Anti-Doping Agency for his role in Lance Armstrong's doping programme.
Following fourth-placed finishes at the Giro d'Italia in 2012 and 2013, Scarponi joined Astana in 2014, primarily riding Grand Tours as a domestique and helping team-mate Vincenzo Nibali to victory in the 2014 Tour de France and 2016 Giro d'Italia.