World champion Tom Boonen has pulled out of the Tour de France, organisers said on Tuesday.

Boonen stepped off his bike 59km from the finish of the 15th stage over 187km from Gap to l'Alpe d'Huez.

The Belgian Quick Step rider, who was second in the standings for the best sprinter's green jersey, failed to win a stage but wore the overall leader's yellow jersey for three days.

"Tom does not understand what happened to him," Quick Step team spokesman Alessandro Tegner said by telephone.

"He passed the Col de l'Izoard with the favourites and suddenly, on the descent, he had trouble breathing.

"He could not control it. He lowered his pace and when he was trying to step up a gear, it was worse. He was powerless, he could not go on."

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