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Tour de France leader Chris Froome has said he is on a "personal mission to show the sport has changed" after insisting he is clean of drugs.

The Briton emphatically won Saturday's first mountain stage of the 100th Tour by 51 seconds to take the race leader's yellow jersey for the first time.

When asked to confirm he had not taken any banned substances, the Team Sky rider, 28, said: "One hundred percent.

"I know the results I get are not going to be stripped 10 years down the line."

It was inevitable that Froome would face questions about doping in light of Lance Armstrong's admission earlier this year that he used performance-enhancing substances throughout his seven successive Tour de France victories from 1999 to 2005.

Britain's Sir Bradley Wiggins faced similar questions after taking control of the yellow jersey on his way to victory in 2012 and while he reacted angrily to the line of questioning, Froome remained calm.

"I think it's normal that people ask questions in cycling given the history of the sport," he said.

"I know the sport has changed. There's absolutely no way I'd be able to get these results if the sport had not changed.

"Results now are definitely a lot more credible. The questions should be asked about people who were winning races maybe five, 10 years ago when we know doping was more prevalent.

"For me it is a bit of a personal mission to show that the sport has changed.

"Anyone who actually spends a bit of time with the team will see that this is months and months of preparation - going to these training camps at altitude all together.

"The support off the bike from the sport staff, from my fiancee, there is so much preparation that it's not 'wow', it does add up."

Earlier, Froome had called his Tour de France stage eight win "a dream come true".

He surged ahead of his main rivals 5km from the end to finish 51 seconds ahead of Team Sky team-mate Richie Porte, while Spanish rivals Alberto Contador and Alejandro Valverde and Australian Cadel Evans struggled when Froome made his move for the lead.

Valverde finished 68 seconds adrift of Froome in third place. Two-time Tour winner Contador (Saxo) finished one minute, 45 seconds behind Froome in seventh, while 2011 champion Evans (BMC) was more than four minutes off the pace.

"I couldn't have asked for more," said Froome.

"Now we've got the yellow jersey we've got to defend it."

He took the yellow jersey from South Africa's Daryl Impey, whose challenge faded on the first of the stage's two mountain climbs.

Froome, who won stage seven of last year's Tour to put team-mate Wiggins in a yellow jersey that he retained all the way to the finish in Paris, was surprised by the gap in times.

"I wasn't expecting that much," added Froome, who is trying to become the second British winner of the race, in its 100th edition.

"I can only thank my team for the opportunity today and for their hard work.

"We're in a really good position now, but we're going to have our work cut out for us over the next two weeks for sure. It's a long way from being decided."

Stage eight results:

1. Chris Froome (GB/Team Sky) 5:03:18"

2. Richie Porte (Aus/Team Sky) +51"

3. Alejandro Valverde (Spa/Movistar) +1:08"

4. Bauke Mollema (Ned/Belkin) +1:10"

5. Laurens ten Dam (Ned/Belkin) +1:16"

Overall standings:

1. Chris Froome (GB/Team Sky) 32:15:55"

2. Richie Porte (Aus/Team Sky) +51"

3. Alejandro Valverde (Spa/Movistar) +1:25"

4. Bauke Mollema (Ned/Belkin) +1:44"

5. Laurens ten Dam (Ned/Belkin) +1:50"