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Michael Phelps, the most successful Olympian in history, has given further indication of a comeback by re-registering with the United States Anti-Doping Agency.

The 28-year-old American retired from competition after winning his 18th Olympic gold medal during London 2012.

But Phelps has returned to training under Bob Bowman at North Baltimore.

Having completed doping tests, he will be eligible to compete in trials for the 2015 World Championships.

Rumours have circulated for some time that Phelps is keen to compete at the Rio Olympics in 2016.

It would be his fifth Games where, in addition to his 18 golds, he has also won two silver and two bronze medals.

Responding to the speculation about a return he said: "Nothing is set in stone. If I decide to keep going and swim again, then I'll compete.

"If I don't, I guess I'll re-retire. Just don't compare me to [American football quarterback] Brett Favre [who made several comebacks]."

Coach Bowman was also non-committal on the subject of Rio 2016. "I just think he's in a place where he's feeling good about swimming,'' he said.

"If he chooses to compete, he's got some time. To be perfectly honest, he's not anywhere near being able to compete in a meet or anything like that. We're just getting started on improving his fitness. We'll see where that goes."