Race officials have also declared the ruling a commonsense decision after concerns were raised that anti-doping regulations might delay the Texan's comeback plans.

''I am pleased and thankful with the UCI decision handed down today,'' Armstrong said in a statement after the world governing body's move to allow him to compete in January.

Doubts were raised over Armstrong's comeback starting in Adelaide when it was revealed he would not be subject to drug testing for the six months required under UCI rules.

A strict application of dope testing provisions would not have allowed the 37-year-old, seven times Tour de France champion to compete until February 1, 2009, six months after he filed paperwork with the US anti-doping agency.

But the UCI said Armstrong could return early because its drug-testing standards had improved.

A relieved Tour Down Under race director Mike Turtur said the decision had ended a stressful wait for race organisers and the entire cycling community in Australia.

''It's been a great decision based on good facts,'' he said.

Armstrong's comeback is meant to draw attention to his global campaign to fight cancer, a disease he famously survived before winning seven straight Tours from 1999-2005.

It is also a defiant stand against critics who publicly doubt he could have achieved those victories without performance-enhancing drugs.

Now he is liable to be tested at any time without notice and will have his own biological passport as part of a UCI-backed scheme to monitor possible doping offences.

Riders must give a series of blood and urine samples which allow a World Anti-Doping Agency-accredited laboratory to measure abnormal fluctuations in body readings.

The American said last month he was tested in late August, and has enlisted a personal anti-doping expert, Don Catlin, who will make his test results public.

Turtur said despite Armstrong's critics the facts surrounding his racing career spoke for themselves.

''He's never produced a positive test and none of the riders that have been part of his team for that seven-year period on the tour have ever returned a positive test,'' he said. AAP