Simon Yates reached out and grabbed a slice of Tour de France glory with victory on Thursday's stage 12 from Toulouse to Bagneres-de-Bigorre.

The 26-year-old Vuelta a Espana champion is riding the Tour to help his twin brother Adam in the general classification but when an opportunity for a stage success came his way in the Pyrenees, he did not pass it up.

"I've been saving energy until we got here in the mountains and this was the first chance to try something," said Simon, who became the 14th different Brit to win a Tour stage and completes the set with victories in all three Grand Tours.

"Normally I would be back helping Adam but I had my own chance and grabbed it with both hands."

What was only the second mountain stage of this Tour enticed a 40-man breakaway up the road, but after they crested the Col du Peyresourde, it was Simon who did much of the work to break it apart on the second of two category one climbs, the Hourquette d'Ancizan.

The Mitchelton-Scott man rode clear with Bora-Hansgrohe's Gregor Muhlberger before they were joined by Pello Bilbao of Astana on the long descent into town.

The Lancastrian looked perhaps the least likely of the three to win in a sprint given his slight frame, but he used some of his old track racing nous to attack on the way into a corner with 200 metres to go and got the power down on the final straight.

"I wasn't super confident in my own sprint," he said. "But you never really know after such a long day how fresh the other guys are, how fast."

Simon has insisted throughout he is only here to help Adam but there was always a sense he might look for a stage or two along the way.

"There were always going to be opportunities in these sort of stages, especially when the breakaway is so big," he said.

"This is probably one of two opportunities total I will have so I was just thankful that I was able to pull it off.

"Now it's back to the day job in the coming days, looking after my brother."

Adam – seventh overall – had a beaming smile on his face as he rolled into town in the peloton some 10 minutes later.

The top of the general classification was unchanged on a day when Julian Alaphilippe was able to stay with the main contenders to retain the yellow jersey and his 72-second advantage over Geraint Thomas.

"I was expecting that maybe some riders can attack but at the end everybody was quiet, it's not bad," said Deceuninck-Quick Step's Alaphilippe, who is not expected to keep yellow through the mountains to come this weekend but will have the honour of wearing it on Friday, the 100th anniversary of it first being awarded.

Thomas admitted he too expected attacks, but Team Ineos were never going to be the ones to initiate them as they hold second and third in the standings.

"Everyone was rock steady and nothing really happened," said Thomas. "We were happy with that. Now it's about the next big three days.

"Everyone is mindful that the next three days are going to be big for the GC and they're waiting for that."

The weekend brings Saturday's summit finish on the Tourmalet, but major movements in the standings are expected in Friday's 27km time trial around Pau.

It is a clear opportunity for Thomas to pick up time on Alaphilippe, and he may now be favourite to win the stage after the bizarre exit from the race of reigning world time trial champion Rohan Dennis.

The Australian climbed off his bike at the feed zone midway through the stage, with his Bahrain-Merida team at a loss to explain why.

There were suggestions Dennis had been unhappy with his time trial equipment and had been arguing with the team, but sports director Gorazd Stangelj said he was not aware of an issue, while confirming Dennis has no physical injury.

"We're also confused," he said. "I'm disappointed about what happened with Rohan today because we expected the effort from him tomorrow.

"It was his decision to stop today in the feed zone. We tried to speak with him. We stopped the car and tried to find a solution to what's going on.

"But he just said, 'I don't want to talk,' and abandoned the race."