Phelps, winner of the event at the past two Olympics, had said after the 2008 Beijing Games that he wouldn't swim it again in major competition.

Phelps will set a match-up against Ryan Lochte in the 400m medley on Monday, which is the first event of the eight-day meeting.

Phelps holds the world record in the 400m medley at 4:03.84, set winning the gold in Beijing. Lochte settled for bronze in Beijing behind Phelps and Hungarian Laszlo Cseh, but claimed the world title in Shanghai worlds last year.

"I love racing against him," Lochte said. "He's one of the hardest racers in the world. He'll go toe-to-toe with you until the end. That's excitement for me. I really hope he does swim that."

Phelps is entered in seven events at the eight-day trials, where the top two finishers in each event book an Olympic place.

That raises the prospect he could vie for 10 medals in London.

"We've done a lot of cool, amazing things and now it's just time to have fun," Phelps said. "We're more relaxed and we'll see after this week what size cherry I want to put on top of my sundae."