Team Ineos manager Dave Brailsford confirmed the incident in Roanne, central France, which occurred during a reconnaissance of the day’s individual time trial.
Froome is understood to have lifted his hands off his handlebars to clear his nose when he lost control of his bike on a fast descent, crashing into a wall.
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He was treated by an ambulance on the scene before being airlifted to hospital. His bike was later spotted being returned to the team truck with damage to the right handlebar and its chain removed.
“He crashed in the downhill section of the course at high speed,” said Brailsford. “He hit a wall. The ambulance came quickly, he’s been taken care of and waits for an helicopter to be transferred to Lyon or Saint-Etienne.
“It’s a very serious accident. Clearly, he won’t be at the start of the Tour de France. It’ll take quite a long time before he races again.”
His team-mate Wout Poels, who was travelled with Froome at the time, said they were riding at 65kph when the crash occured.
“We were after the climb on the downhill and the wind took Chris Froome’s wheel and he went down like really, really hard on the floor, and I think we were going 66 or 65kph. It was really bad,” Poels said.
“It was pretty windy out there and especially on a TT bike. I mean especially with the high wheels and you’re also in a little bit of a less comfortable position of course. Yeah, it can happen. If you go down at 65kph, and he went down really, really hard, then you know it’s not really good.”
Full recovery from femur fractures typically take several months, and Froome is set for a long road to recovery which is likely go beyond the end of the current season.
Earlier in the day, Brailsford had said he would do everything in his power to help Froome, 34, achieve his goal of joining an elite band of four riders to have won five Tours de France.
Instead Team Ineos will now focus their attentions on last year’s yellow jersey winner, Geraint Thomas, and the talented young Colombian Egan Bernal who played such a crucial role in helping Thomas to victory.
Aged 22, Bernal was set to lead the team at the Giro d’Italia before a crash on a training ride in his home country of Colombia forced him to pull out, and reset his sights on the Tour de France.
Bernal and Thomas will both race the Tour de Suisse, which starts on Saturday.