ROME (Reuters) - Italy's 2008 Olympic 50km race walk champion Alex Schwazer, who returned from a doping ban to win the world championship title last month, has again tested positive for a banned substance, Italy's athletics federation said on Wednesday.

Schwazer, appearing at a news conference with his trainer, lawyer and manager, vigorously protested his innocence after returning a positive result for a steroid after a re-test.

"This is not like four years ago. This time I don't have to apologize because I have done nothing wrong," he said. "Probably someone doesn't want me to go to the Olympics."

Schwazer, who won the European 20 km title in 2010, served a ban of three years and nine months after admitting using the EPO blood-booster in 2012, and now faces a life ban from the sport.

He told reporters he was still determined to participate in the Rio Olympics even though he realized he had little time left to clear his name.

"This is a nightmare for me because I have invested so much in winning (in Rio) and I am still investing so much, but I won't give up because I am facing hostility," he said.

Schwazer's trainer Sandro Donati said he would also "fight to the end" to prove his athlete's innocence.

Donati said Schwazer had tested only marginally positive for a steroid which, in any case, would benefit an athlete requiring explosive strength in a sport like sprinting, not stamina and resistance as in the case of a 20 km and 50km walker.

He said it was also strange that a sample on which Schwazer had tested negative for banned substances in January was re-tested and found positive in May, though he added he had no doubts about the reliability of the test itself.

Before Schwazer's 2012 ban and exclusion from that year's London Games, he was a hero in Italy after winning Olympic gold, beating a 20-year-old record set by Russian Vyacheslav Ivanenko by more than a minute.

(Reporting by Isla Binnie and Gavin Jones, editing by Mitch Phillips and Ken Ferris)