The 100th edition of the race begins Saturday in Corsica.
Armstrong, who had his seven Tour wins between 1999 and 2005 stripped from him after he admitted doping, told the French newspaper Le Monde the race could not be won during his heyday without erythropoietin blood doping, which adds oxygen to the blood, Radio France Internationale reported Friday.
"The Tour de France? No. Impossible to win without doping because the Tour is an endurance event where oxygen is decisive," Armstrong said.
The report did not specify if Armstrong meant doping is still needed to win.
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