91Èȱ¬

Eighteen-time Olympic swimming champion Michael Phelps says concerns about weight gain were behind his decision to return to the sport.

The 28-year-old American ended his career after winning an unprecedented 22nd Olympic medal at London 2012, but confirmed his competitive comeback earlier this month.

He will swim the 100m butterfly event at the Mesa Grand Prix on Thursday.

"Getting back into shape was the number one thing," he said.

"At my highest point, I was 225lb [102kg] and I raced at 187lb [85kg] in London, so the weight came on pretty quick."

Ahead of his return in Arizona, North Baltimore-based Phelps continued: "I wanted to see how much weight I could lose and how much I could progress."

Long-term coach Bob Bowman admitted the most decorated Olympian was "really out of shape" when he returned to the pool last year but the swimmer has made encouraging progress since.

Phelps fuelled speculation about a potential comeback in November by re-registering with the United States Anti-Doping Agency.

Bowman told the 91Èȱ¬ soon after that he believed Phelps could challenge for further honours at the 2016 Rio Games if he was fully motivated.

Phelps himself was non-committal over his long-term future at a news conference on Wednesday but admitted to enjoying the sport more than he did leading into the London Olympics.

"Going into 2012, it was hard," he said. "There were lots of ups and downs and it was hard to get motivated at times."

Phelps dismissed concerns he could damage his legacy if his comeback failed.

Five-time Olympic champion Ian Thorpe tried to come back ahead of London 2012 but did not qualify.

"I'm doing this for me," said Phelps. "If I don't become as successful as you all think I should be, then that's your own opinion."