After winning six titles at Athens 2004 and an unprecedented eight at Beijing 2008, Phelps won four more at London 2012 before announcing his retirement. 

At first he insisted that he there was no prospect of him returning to action and he even joked that if he was to compete at Rio 2016, it would be in the new Olympic discipline of golf rather than swimming.

However there has been much talk that he may be changing his mind and the announcement of his return to doping tests gives substance to the reports.

Responding to the latest speculation Phelps revealed that "nothing is set in stone" but added, rather deceptively, that "if I decide to keep going and swim again then I'll compete but if I don't, I guess I'll re-retire."

Michael Phelps has been enjoying other activities since announcing his retirement after London 2012 ©Getty Images


The International Swimming Federation (FINA) requires an athlete to be tested for at least nine months before taking part in sanctioned events such as the World Championships.

By subjecting himself to drug testing at this stage, Phelps has given himself plenty of time to go through an entire season, including the US trials, before the Championships. 
  
This will delight spectators as well as swimming officials keen to profit on the publicity that the most medal-winning Olympian in history brings, although the possibility may not be considered quite as enthusiastically by his rivals.

Phelps would be 31 by the time of the Opening Ceremony at Rio 2016 but, in the modern era, that is not particularly old and US teammate Dara Torres won three silver medals at Beijing 2008 aged 41 so a whole decade older.

Although coming out of retirement is associated more with non-Olympic sports such as professional boxing, it has been seen in the past.

Among those who have successfully returned to win Olympic gold is the British rower Sir Steve Redgrave - who won his fifth straight title at Sydney 2000 after briefly retiring after his fourth in Atlanta 1996.

Steve Redgrave is one Olympian to postpone retirement plans...despite earlier saying "anybody who sees me in a boat has my permission to shoot me" ©Getty Images


Phelps' longstanding coach Bob Bowman was non-committal on the subject of Rio 2016, but certainly did not rule it out.

"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 and we'll see where that goes."

"This may not go anywhere and we don't have an event picked out or anything like that and there's no grand scheme.

"It just sort of makes sense that he can make some choices if he wants to."