* Phelps retired from swimming after 2012 London Games
* Had repeatedly said he would not contemplate a comeback
Michael Phelps, the most decorated Olympian of all time, is coming out of retirement, USA Swimming said on Monday.
The American, who has not competed since the 2012 London Games, will headline an April 24-26 meet in Arizona along with fellow Olympians Ryan Lochte and Katie Ledecky.
Phelps, who turns 29 in June, rejoined the U.S. Anti-Doping Agency's drug testing programme last November, a clear signal that he could be considering a return to the swimming pool in time for the Rio de Janeiro Games in 2016.
Phelps retired from competitive swimming after winning four gold and two silver medals at the 2012 London Olympics to take his career tally to 22, 18 of which were gold.
He had repeatedly said before the London Games he would not contemplate a comeback after he retired and his coach downplayed any suggestions Phelps was considering a return to compete at what would be his fifth Olympics in Rio.
The Next Arena Grand Prix at Mesa is the fourth stop of USA Swimming's 2013-14 Arena Grand Prix Series, which provides the nation's best swimmers six opportunities to face top-flight competition on U.S. soil.