Michael Phelps became the most decorated Olympian of all time on Tuesday, winning a record 19th medal in the pool.

Phelps anchored the US team to a resounding victory in the 4x200m freestyle relay to secure gold and beat the record of 18 medals amassed by Soviet gymnast Larisa Latynina between 1956 and 1964.

The American had earlier been thwarted in his bid for a 200m butterfly Olympic treble by Chad le Clos of South Africa, but a silver medal allowed him to match the record medal haul before the relay sealed his place in history.

Phelps, 27, won six golds and two bronze medals at the 2004 Athens Games, then collected a record eight golds in the Beijing Games.

He had suffered a testing start to the London Games, slumping out of the medals in the 400m individual medley and then having to settle for silver in the 4x100m freestyle relay.

But Phelps' unprecedented 19th medal was never in doubt in Tuesday's relay as he took over the final leg with a clear lead built up by his teammates and powered home to leave France trailing in silver and China with bronze.

"I think the biggest thing I have always said is anything is possible," Phelps said of breaking Latynina's mark. "I put my mind on doing something no one has ever done before and nothing was going to stand in my way."

Turning to his relay colleagues, Phelps said, "I just wanted to thank them for allowing me to have this moment."