Armstrong (left) and Pantani had a bitter rivalry

The pair staged an intense battle during the 2000 Tour de France, which Armstrong won.

"This is terrible and shocking news," said the American, who fell out with Pantani over events in that Tour.

"Regardless of our battles on and off the bike, I had a deep respect for Marco. Cycling has indeed lost a great champion and a great personality."

Cycling mourns Pantani

Armstrong joined a long list of cycling celebrities in mourning Pantani's death.

"My thoughts and condolences go out to his family, friends, and fans," he said.

Tour de France director Jean-Marie Leblanc denied Pantani entry into the last two Tours, following a failed drugs test.

But he admitted that the Italian "marked the history of cycling by being the last great climber of the generations of climbers that we've known".

Former world champion Laurent Jalabert added: "Undoubtedly it hit him hard when he was thrown off the Tour of Italy in 1999. He probably did a few things he shouldn't have, but he didn't get the chance to kickstart a career off the bike."

Jalabert added: "But Pantani was a genius... It wouldn't be fair to summarise his life with issues of doping and depression."