Lance Armstrong makes "Final Tour de France" announcement on Twitter; bid to win eighth title starts in Rotterdam on Saturday.

Seven-time Tour de France winner Lance Armstrong (Team RadioShack) has announced that he is making his final appearance in the Tour next month.

The 38-year-old American, two years into a second comeback, revealed the news through his Twitter website just days before he begins his campaign for an unprecedented eighth victory in the gruelling event.

"And yes, this will be (my) final Tour de France. It's been a great ride," Armstrong tweeted and added "Looking forward to three great weeks."

Armstrong overcame testicular cancer and a break from professional cycling while undergoing treatment to win the Tour de France seven times in a row between 1999 and 2005.

He retired in 2005 but made a comeback last year, at age 37, saying he partly missed the thrill of competition but was driven by a greater cause, to help promote cancer awareness.

He finished a creditable third in last year's Tour de France, behind his Spanish teammate Alberto Contador, but formed his own Radioshack team for this year's race after the pair fell out.

But Armstrong's comeback has not been well-received by everyone. He has been dogged by accusations of doping throughout his career and although he has never failed a test and always maintained his innocence, the claims have continued.

Just last month, his former team mate Floyd Landis, who was stripped of his win in the 2006 Tour de France because of a doping offence, accused Armstrong and other members of his team of cheating.

The latest accusations prompted a flurry of finger-pointing and counter-accusations with Armstrong hitting back at Landis, who finally owned up to using performance-enhancing drugs after years of denials.

"It's our word against his word," Armstrong said at the time. "I like our word. We like our credibility. Floyd lost his credibility a long time ago."

This year's Tour de France starts with a prologue in Rotterdam on Saturday and ends in Paris on July 25.

Contador remains the favourite to win but Armstrong is regarded among the main contenders.