Michael Phelps can win more Olympic medals - Bob Bowman

Michael Phelps can challenge for medals at the 2016 Rio Olympics if he decides to return to competitive swimming, says his long-term coach Bob Bowman.

American Phelps, 28, retired after winning an unprecedented 22nd Olympic medal at the London Games last year.

"I have no doubts he can physically and mentally do it [win medals]," Bowman said of the 18-time Olympic champion.

He revealed Phelps signed up for drug-testing in May, which would make him eligible for competition in March 2014.

18-time Olympic champion Michael Phelps

Phelps speaks after his final race at London 2012

Bowman coached Phelps for 16 years, beginning when the 18-time Olympic medallist was only 11 years old, and their time together included Beijing 2008 when Phelps claimed eight gold medals - the most ever recorded in a single Games by an individual.

Following those achievements Phelps occasionally struggled with motivation upon his return to the sport in 2010.

Bowman admitted Phelps had not put in enough distance training for the London Olympics after the American could only finish fourth in the defence of his 400m individual medley crown.

Phelps went on to win six medals at the Games, including individual titles in the 100m butterfly and 200m individual medley.

After bowing out as the most successful Olympian of all-time, Bowman confesses he was surprised when Phelps re-registered with the United States Anti-Doping Agency for out-of-competition doping tests.

"I absolutely thought London would be the end," said Bowman. "It was a stretch for him to get there emotionally, psychologically and physically, so I'm surprised he might have an interest in doing something."

Michael Phelps's career record

58 gold medals: 18 Olympic, 27 World Championship, 13 Pan Pacific

11 silver medals: 2 Olympic, 6 World Championship, 3 Pan Pacific

3 bronze medals: 2 Olympic, 1 World Championship

Bowman said that despite the impressive achievements of American World Championship medal-winning swimmers Ryan Lochte, Missy Franklin and Katy Ledecky - as well as South Africa's Chad Le Clos - no-one has stepped up to fill the void left by Phelps's departure.

"Swimming needs him because he shines a spotlight on the sport and adds a level of excitement we don't get from anyone else," added Bowman.

"When we've gone to some meets this year and he is not swimming, you see hardly anyone watching. The stands were full when he was swimming."

While excited about the prospect of Phelps making a comeback and appearing at a fifth Olympics, Bowman, who was speaking at the UK Sport World Class Performance conference,  is also urging caution.

"The two of us have a standard," he said. "Should he choose to comeback, I would never allow him to be in a meet unless he could compete near his top-level. That's just the way we do business."

Michael Phelps

Phelps wins third gold medal at London 2012

Bowman added Phelps would need "years of work" before he could be "competitive at the top" again, but said he was in a good place mentally.

"He didn't have the enthusiasm for the last five or six years, but now he does and that's good to see," said Bowman. "There's no pressure, we're just seeing how he gets on."

Bowman, who was an assistant coach for the US swimming team at London 2012 and aided British Swimming with a post-Games analysis of into their disappointing medal haul, says he will only allow Phelps to return to competition if he is reaching peak fitness.

Phelps made his Olympic debut in Sydney in 2000 as a 15-year-old and won six golds and two bronze medals in Athens, followed by a historic eight gold medals in Beijing.