Former American cyclist Lance Armstrong has revealed he first started doping from the age of 21, during his first season as a professional.

"Wow, straight to the point, probably 21," Armstrong replied when asked how old he was when he first doped as part of an ESPN documentary.

The exchange with US journalist Marina Zenovich appeared in a 90-second trailer released on Monday for a two-part documentary called "Lance" which will be broadcast by ESPN on May 24 and 31.

During the clip, in which former US Postal Service team-mates Tyler Hamilton and George Hincapie respond to the same question about performance-enhancing drugs, Armstrong, now 48, explains there are "a bunch of ways to define doping".

"The easiest way to define it is breaking the rules. Were we getting injections of vitamins and other things like that at an earlier age? Yes, but they weren't illegal. I always asked (what I was being given). I always knew, and I always made the decision on my own," he said.

"Nobody said, 'Don't ask, this is what you're getting.' I never, ever would have gone for that. I educated myself on what was being given, and I chose to do it."

Armstrong dominated professional cycling in the 2000s and won the Tour de France seven years in a row from 1999 to 2005.

He was later stripped of those titles and received a lifetime ban from the sport in 2012 after the US Anti-Doping Agency determined he was the key figure in a sophisticated doping programme on the US Postal Service team.

In 2013, he confessed to doping starting in 1996 in a televised interview with US chat-show host Oprah Winfrey.

His latest admission could also cast doubt over his world road race title won in Oslo in 1993, having turned professional the previous season.