By Julien Pretot

BAGNERES DE LUCHON, France (Reuters) - Briton Chris Froome stunned his passive rivals to snatch the overall leader's yellow jersey in the Tour de France when he won the eighth stage with a lightning attack on the final descent on Saturday.

The Team Sky rider sped away from a group of overall favorites at the top of the last of four grueling climbs in the Pyrenees and never looked back as he raced down at breakneck speed towards the line.

The defending champion, who is chasing a third Tour title, finished 13 seconds ahead of Ireland's Dan Martin (Etixx-Quick Step) and Spaniard Joaquim Rodriguez (Katusha).

Last year's runner-up Nairo Quintana (Movistar), who failed to attack on a searing hot day, was in the group featuring Martin and Rodriguez.

Double champion Alberto Contador lost further ground after being dropped near the top of the Col de Peyresourde, the last ascent of the 184-km stage.

Overall, Froome leads fellow Briton Adam Yates (Orica-Bike Exchange) and Rodriguez, who are second and third respectively, by 16 seconds.

"It was a bit of fun. I thought I would give it (attacking downhill) a try. That was real old-school bike racing," said Froome.

Expected to attack on the climbs, the Briton took his opponents by surprise.

After reaching the top of Peyresourde first, Froome pedaled away as Quintana and others failed to react promptly, taking risks on the descent as he put his whole weight onto the front of his bike in a position reminiscent of Graeme Obree in the 1990s.

Quintana, Froome's main rival, looked comfortable on the climbs but never attacked and his passivity at the top of Peyresourde cost him dearly.

The Colombian is sixth overall 23 seconds off the pace before Sunday's biggest Pyrenean stage up to Andorra Arcalis, where Froome is widely expected to try and hammer the opposition with one of his trademark accelerations.

"The most important (thing) is to gain time when you can. It's done," said Team Sky sports director Nicolas Portal.

Frenchman Thibaut Pinot (FDJ), who dropped out of contention after cracking on Friday, attacked early on the ascent to the Col du Tourmalet but he and his breakaway companions, Pole Rafal Majka (Tinkoff) and German Tony Martin (Etixx-Quick Step) were caught with about 50km left.

There were a few accelerations on the final climb but nothing violent enough to drop Froome, although one of them was fatal to Contador, who slipped further down the rankings after being injured in crashes in the first two stages.

"I did everything I could," said Contador, a Tour winner in 2007 and 2009 who is now 3:12 off the pace.

"It's terrible that I've crashed like this at the beginning of the Tour. That's cycling, danger is everywhere."

(Editing by Rex Gowar)