Alpe d'Huez - Former champion Vincenzo Nibali was doubtful for the remainder of the Tour de France after a collision, allegedly with a police motorbike, on Alpe d'Huez left him nursing a "suspected broken vertebra" in his back.

Nibali, who claimed his sole yellow jersey in 2014, was chasing Frenchman Romain Bardet (AG2R) on a narrow stretch of the road lined with thousands of enthusiastic fans when he suddenly came crashing down four kilometres before the summit of the legendary climb.

The Bahrain team rider was helped back on his bike and, with a courageous effort, rejoined the chase of a small group of stage leaders in a thrilling finale that saw Britain's Geraint Thomas triumph for the second alpine stage in succession.

He eventually finished in seventh place, only 13 seconds behind race leader Thomas, who took his overall lead over four-time champion and teammate Chris Froome to 1:39.

Nibali is fourth overall at 2:37 behind Thomas, but was later taken to hospital in Grenoble where he was being treated for a suspected broken bone in his back.

The Italian later blamed a police motorbike for his spill.

"Bardet had a 10 second lead on us and we were in amongst the motorbikes," he explained.

"At that point the road narrowed, there were two police motorbikes close by.

Chris "Froome accelerated, I followed him. Suddenly, it slowed down in front of me, and I hit the deck.

"I was feeling good, I really believed in my chances. It's a pity. But these kinds of things can happen when there's a lot of people on the road."

Thomas said: "I was on his (Nibali) wheel, but I didn't see if he hit a spectator. I rode over his back wheel and managed to get back on.

"It's not good to see, because he should have been there challenging for the stage. But it's just fortunate that we were ok."

The profile of Friday's 13th stage could help the Italian, a former two-time winner of the Giro d'Italia, if he decides to continue.

It is mainly undulating 169.5km race from Bourg d'Oisans to Valence.