Cyclist Daan Myngheer, 22, has died in hospital after suffering a heart attack during the first stage of the Criterium International in Corsica.
He is the second Belgian cyclist to die in as many days after Antoine Demoitie was struck by a motorbike during Belgium's Gent-Wevelgem race.
Myngheer, who turned professional last year, had been in an induced coma.
His team, Roubaix Metropole Lille, said in a statement: "He lost his last race after fighting like a champion."
Myngheer pulled up in difficulty 20 miles from the line in the first stage of the race at Porto-Vecchio on Saturday.
He suffered the heart attack while in an ambulance, before being transferred by helicopter to hospital, where he was put on artificial respiration and fell into a coma. According to his team, Myngheer's organs have been donated.
British cyclist Mark Cavendish tweeted: "Such a terribly sad few days in the professional cycling world. Another tragic loss.
"RIP Daan Myngheer. My thoughts are with your loved ones."
Team Sky added: "Our thoughts are with the family and friends of Daan Myngheer. So sad to lose another member of the cycling family."
Welsh cyclist Owain Doull wrote: "Tragic news to wake up the death of Daan Myngheer. I've been racing against Daan since I was 15 and always looked up to him. Gone too soon."
Tour de France organisers ASO, who also operate the Criterium International, said on Twitter: "Our deepest condolences go out to the families, friends and team-mates of Antoine Demoitie and Daan Myngheer."
UCI president Brian Cookson wrote on social media: "My thoughts are with the family, friends and teammates of Daan Myngheer."
The latest tragedy comes after the 25-year-old Demoitie, who was riding for the Wanty-Gobert team, was killed when he was hit by a motorbike.
The accident happened when several riders crashed as they passed through northern France during Sunday's Gent-Wevelgem one-day race.
Demoitie was taken to hospital in Lille but died some time later.
A minute's silence was held for both riders at the beginning of the Three Days of De Panne on Tuesday morning.
In January, six racers from the Giant-Alpecin team, including German John Degenkolb and Frenchman Warren Barguil, were injured in a collision with a car during a training ride south of Valencia.