Lance Armstrong: I'm to blame for doping slurs against Chris Froome

American Amstrong, 43, was stripped of his seven Tour titles in 2012 for using performance-boosting drugs, but hardly helped the 2013 winner Froome when he made a comment on Twitter after the English cyclist had won stage 10 of this year's event on Tuesday.

Froome said on Wednesday he is willing to undergo physiological tests after this year's race to put to rest any suspicion of doping.

"Too strong to be clean?," tweet Armstrong. "Don’t ask me, I have no clue.”

Heading to France to take part in a charity ride, Armstrong refused to apologise for that tweet, but did defend Froome saying he should not be forced to deal with constant questioning on the subject because of his own conduct.

“Anything regarding a bike race is going to be closely scrutinised and questioned," said Armstrong on Sky TV. "I get those questions all the time, and the question is: ‘Can this be believed? Are these guys clean? Is everybody like you Lance?’

“I know what it’s like for a guy like Chris to be in the middle of a Tour to deal with the constant questions which of course he is, and to be fair and to be honest to him a lot of that is my fault.

“Whoever is winning the Tour de France in 2015 should not be answering questions about somebody that won it 10 or 15 years ago. That’s just not legitimate, that’s not true.”

Meanwhile, Froome said Armstrong's controversial presence on the route of the Tour de France as part of a charity ride is a non-event.

Armstrong is taking part in Geoff Thomas's 'One Day Ahead' operation and will join former England footballer Thomas and a team of amateurs, who are aiming to raise one million pounds ($1.57 million) for Cure Leukaemia, by cycling in two stages a day before the official peloton goes through.

Armstrong will be at the start of a ride from Muret to Rodez on Thursday while the actual Tour peloton will be in the mountains on the 12th stage going up to the Plateau de Beille.

"We definitely don't see this as him being back on the Tour," Froome told a news conference.

"You've got to look up to what Geoff Thomas is doing because as I've said earlier in the race, I support his cause. It's a cause that is close to my heart.

"I wish Geoff Thomas and the guys all the best in raising as much money as they can but as I said about Lance he's not going to ride with us, it's a non-event."

International Cycling Union (UCI) president Brian Cookson had already described Armstrong's presence on the ride as 'disrespectful'.