The Australian rider threw in the towel at the start of the Peyresourde, the first of two climbs on the stage, after he had been busy trying to make the early breakaway.

His retirement comes on the eve of Friday's time trial in Pau where the current world champion and former one-hour world record holder would have been one of the favourites.

"We are as surprised as you are. We are also disappointed. We were expecting a great performance from him tomorrow in the time trial. It was his decision to stop at the feed zone," said Gorazd Stangelj, the team's sports director, after the stage.

Dennis was being primed as a Grand Tour contender who could pulverise time-trials and defend himself on the climbs, as he did when coming second on the recent Tour de Suisse.

"We tried to talk to him, we stopped the car to discuss a solution. He told us he didn't want to talk and got in the car," said Stangelj. "I don't know any more than you do."

Stangelj said he did not think it was a physical problem.

"We will have to clarify the situation at the hotel tonight," he said.

The team released a statement saying it had launched an investigation.

Tweets in several languages by broadcaster Eurosport suggested Dennis had an argument with his team over his helmet radio and that his decision to expend energy attempting to join the breakaway the day before a time trial might have added to existing friction.

Australian broadcaster SBS tweeted that the 29-year-old had returned to the team bus, before leaving without comment, accompanied by his agent and a press officer.

After the stage, teammate Dylan Teuns told French TV that he was puzzled after he was the first Bahrain rider to finish.

"When I got to the bus, there was a bike," said Teuns. "It's very surprising because I knew I was the first one on the team. I saw him this morning, he seemed fine. Maybe he's sick, I don't know."

Dennis won the first stage of the 2015 Tour and wore the yellow jersey but has only finished one of his four Tours.