Alberto Contador has insisted that Tour de France leader Chris Froome has been riding clean during this year's race.

Froome, who is part of Team Sky, has a four-minute lead in the 100th edition of the event with six stages remaining.

However, the Brit's performance has been constantly questioned amid allegations of doping.

"There is no reason to doubt Froome," Sky Sports News quotes Contador as saying.

"He is a big professional. He has a very high level. He has been at that level for the whole year. His performance is because of his work.

"There are some times when you are stronger and some times not. I believe in him and am absolutely confident in his performance."

Contador is currently in third place in the overall standings, four minutes and 25 seconds behind Froome.