Richie Porte's Tour de France dreams have been shattered after the Australian was involved in one of the worst crashes in the race's history.

Porte was part of an elite group screaming down the treacherous Mont du Chat descent in the closing stages of an carnage-laden Stage 9 when he lost control on a twist in the road and slammed into the bitumen at 72.5km/h.

The Tasmanian veered onto the roadside dirt where he came off his bike, skidded back across the tarmac and into a rock wall where he was hit by trailing Irishman Dan Martin.

In shocking scenes, Porte lay bloodied and motionless for several minutes before being fitted with a neck brace and loaded into an ambulance. Incredibly, scans have confirmed no broken bones.

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BMC sports director Fabio Baldato said the team was gutted.

"We are all sad. We did a lot of work and Richie did a lot of work, for this Tour de France since the beginning of the year," Baldato said.

"We did everything possible to put him on the podium from the first day. But he can come back stronger because he deserves a good result and a great Tour de France."

Australia's Richie Porte gets medical assistance after crashing in the descent of the Mont du Chat pass during the ninth stage of the Tour de France. Photo / AP.
Australia's Richie Porte gets medical assistance after crashing in the descent of the Mont du Chat pass during the ninth stage of the Tour de France. Photo / AP.

Porte's wife, Gemma, wasn't at the race, but tweeted only hours before her husband's crash.

"Day of travel for me so only informative TdF messages/updates wanted. Nothing to spark the wifey nerves please!"

The mountainous 181.5km stage from Nantua to Chambery was feared as the most brutal day of this year's Tour, but no one could have predicted the destruction to come.

Jesus Herrada, Rafal Majka, Geraint Thomas and Robert Gesink also crashed, with the latter pair forced to abandon.

Thomas broke his collarbone after hitting the deck hard on the second climb, the Col de la Biche, while Gesink came off only a handful of kilometres into the stage.

After getting caught up in the crash with Porte, Martin was able to continue only to crash again further down the same road on the Mont du Chat before eventually finishing in Chambery.

It raised more questions on whether such a dangerous descent should be included in the Tour. The mountain had not featured in the race since 1974.

"I guess the organisers got what they wanted. I don't think anyone want to takes risks, but it was so slippy out there," Martin said.

"Richie just lost it on one corner, locked his back wheel and there was nowhere to go. I was lucky to come away with what I did."

Porte had spoken before the start of the stage about the need to show caution on that stretch of road.

"It's a fast descent, it's quite tricky. It's not the nicest of descents, but you ride to the conditions and if you take big risks there you can also crash," Porte said in the morning. "Hopefully it's a straightforward run into the final."

But by the finish line, his teammate Nico Roche was still in shock.

"Someone shouted out to me from the side of the road, 'Richie crashed, Richie crashed'. I didn't want to believe it," Roche said.

"At the moment he probably doesn't want to believe this has happened. We had done all the reconnaissance and everything. It's the thing with sports, unfortunately. They'll be good moments and bad moments."

It is a devastating blow for Porte, who had grown from domestique to outright leader and was contesting his first Tour as the No. 1 protected rider.

He had ridden superbly right up until the moment of disaster and after nearly 4600m of climbing, was part of a small group of survivors chasing lone leader Warren Barguil up the Mont du Chat.

Fabio Aru appeared to attack Chris Froome after the Brit had raised his hand with mechanical issues. Several raids then followed.

But when Froome launched a scorching acceleration of his own, only Porte, Rigoberto Uran and Romain Bardet could follow before disaster struck on the way down.

Colombian Uran won the stage in a photo-finish with Barguil, despite Uran being unable to change gears after his bike was hit in Porte's crash.

But Froome again retained yellow and, if this show of strength is a sign of things to come, a stranglehold on a potential fourth Tour crown.