The American will join former England footballer Geoff 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 is to go ahead with his plans, said Thomas, even though he was advised not to proceed by UCI (International Cycling Union) president Brian Cookson in March.
"We know Lance's involvement has split opinion so we've tried to be as respectful as possible," Thomas told the BBC on Friday.
"The stages Lance will be riding come towards the end of week two when I know all the (charity) riders will need some support.
"I know his arrival will give them the encouragement they will need to carry on with this gruelling challenge and in turn raise as much money as possible for blood cancer patients."
Armstrong is due to ride stages 13 and 14 on 16 and 17 July but Cookson said earlier this year that his involvement would be unwise.
"Lance would be well advised not to take part in that," he said. "I'm sure Geoff Thomas means well but frankly I think that's completely inappropriate."