Marchand clocked 4:02.50s – more than a second faster than Phelps – to finish ahead of American Carson Foster in 4:06.56s and Japan’s Daiya Seto in 4:09.41s.

Marchand is set to be one of the faces of next year’s Paris Olympics and he warned that “the best is yet to come”.

“It was insane,” the 21-year-old said.

“That was one of the most painful things I did. That was amazing to do it here. The time is crazy.”

Phelps’ record of 4:03.84s had stood since the 2008 Beijing Olympics and was the longest-held world record in swimming.

Second-place Foster said it was “pretty cool” to watch Marchand break it.

“Obviously the goal is always to win and Leon is a great competitor,” said Foster.

“Being the closest person to history was pretty cool, getting a front-row seat to that.”

Marchand, who is coached by Phelps’ former mentor Bob Bowman, swam the second fastest 400m ever at last year’s world championships in Budapest, coming home in 4:04.28s.

The Frenchman insisted yesterday that breaking the record was just “a step in my journey”.