Britain's Chris Froome, wearing the overall leader's yellow jersey, is silhouetted against a cloudy sky prior to the eleventh stage of the Tour de France. Photo: Christophe Ena/AP

Chris Froome believes Thursday's 12th stage of the Tour de France will be key to claiming a fourth overall title.

The 32-year-old Briton leads the race by 18 seconds from Fabio Aru but the overall contenders face a daunting task ahead of them.

"It's a tough day, over 200km, we go over some big climbs - the Port de Bales is the most significant climb of the day, the third from final one," said Froome.

"We go down just before the final climb... and then 4km up to the finish."

It's a 214.5km Pyrenean mountain stage from Pau to Peyragudes that includes six categorised climbs, with riders expected to spend more than six hours in the saddle.

Five years ago Froome finished second to Spain's Alejandro Valverde on the 17th stage finish at Peyragudes, a stage which Froome believes he could have won had he not been ordered to wait for his then-team leader Bradley Wiggins.

"It's a bit different to 2012 this time. The finish is on an airstrip, an uphill airstrip with an over 20 percent gradient.

"It's savage! If someone blows over those last few hundred metres, there could be some significant time gaps."

July 13 also marks an important anniversary in British cycling as it is 50 years since the death of Tom Simpson during the 1967 Tour on Mont Ventoux due to a mix of heat exhaustion and a cocktail of performance-enhancing drugs.

"Certainly Tom Simpson left a legacy on which I'd like to believe a lot of us British guys racing in the Tour de France continue to build upon," added the race leader.

Froome has unfinished business with this stage finish and he admitted to having felt "frustration" in 2012 when ordered to wait for Wiggins rather than chase down Valverde.

He missed out on the stage win by just 19 seconds and certainly appeared to be strong enough to catch the Spaniard.

But this time he's more concerned with holding onto the yellow jersey.

"There are only two uphill finishes now... so it's going to really be one of the key stages to this year's race," added Froome.

He said he will be sticking to second-placed Aru "like glue" as his lead over the Italian is just 18 seconds.

But that might give third-placed Romain Bardet of France the chance to make up his own 51sec deficit.

And he believes the stage is ideal for him.

"There are many queen stages at the Tour de France, and this is an important stage - a beautiful mountain stage with a summit finish," said the 27-year-old AG2R leader.

"I hope it will go well and I'll be able to wage war.

"We'll do our best, it's going to be a marathon stage of more than 200km, it has the kind of profile I like."

The 12th stage will also pass the 2,024th kilometre from a total of 3,540km over the three weeks, with that point being marked by a special ceremony to celebrate Paris's bid to host the 2024 Olympic Games.

Meanwhile, fourth-placed Colombian Rigoberto Uran is riding high after winning Sunday's stage but says the strength of Froome's Sky team makes him hard to beat.

"Sky have a very strong team, the strongest. When there are just 10 riders left, they have three.

"We'll have to see how the race goes in the final."