Porte led a three-man breakaway from the group of main contenders, which also contained race leader Chris Froome (Team Sky) and Trek-Segafredo’s Bauke Mollema.
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As the crowds swarmed the side of the road, the camera motorbike in front of the three riders came to a halt, forcing Porte to crash head first into the back of it.
Up the road, Thomas De Gendt (Lotto-Soudal) had already taken the stage, with the favourites racing five minutes behind to open up time gaps.
Yates provisionally moves into the yellow jersey, but the result may be changed after the incident.