The irrepressible American sliced more than two seconds off his own 400m individual medley world record to hit the wall in 4:06.22 and become the only swimmer to win seven gold at a single world championship.

Teammate Ryan Lochte took the silver in 4:09.74 and Luca Marin of Italy won the bronze in 4:09.88.

Fellow American Katie Hoff shattered a seven-year-old record in the women's 400m medley, touching in 4:32.89 ahead of Russia's Yana Martynova and Australian Stephanie Rice.

In total, 14 world records tumbled in Melbourne.

But Phelps' quest for eight gold medals was cruelly ended when the United States 4x100m medley relay team was disqualified in the heats after Ian Crocker was judged to have left the blocks early.

"Training wise and racing wise, this is probably the best moment I've had," said the 21-year-old, who has set four individual world marks here and one with the freestyle relay team.

"I didn't expect to be two seconds under the world record. I saw at 150m I was 0.1 under the record. It's the last race and I wanted to finish off strong."

His medal tally surpassed the six Ian Thorpe won at the 2001 event in Japan to make Phelps the most decorated swimmer in championship history with 20 medals, taking the mantle from Hackett who has 18.

Australia's Libby Lenton was also firing, grabbing her fifth gold of the meet, the most ever by an Australian woman, by touching first in the 50m freestyle in 24.53.

Sweden's Therese Alshammar was second and Dutchwoman Marleen Veldhuis third.

"Just to finish off the meet with another gold medal is so incredible," said Lenton. "It's definitely pretty special. I am kind of used to having a great meet and then a not so great meet. To have the consistency is fantastic."

With the eight days of swimming wrapped up, the United States easily topped the medal table with 20 gold to Australia's nine as they start the countdown to the Beijing Olympics.