The American, just 75 minutes after her disappointing fifth in the 200m freestyle final, clocked 15mins 37.34secs ahead of teammate Erica Sullivan (15:41.41) and Germany's Sarah Kohler (15:42.91).

It was well outside her own world record time of 15:20.48, but still one of the fastest ever.

The result was never really in doubt. While Ledecky has swum under 15:40 countless times in her career, just one other swimmer, Denmark's Lotte Friis, has ever broken the barrier.

But Sullivan went close, with a powerful final 200m to narrow the gap on her distinguished team-mate in the final stages.

Ledecky went out strongly and was ahead by a body length after 200m, and she never relinquished the lead to add another chapter to her glittering career.

The women's 1500m is one three new events on the Olympic swimming programme this year, along with the men's 800m free, and the mixed 4x100m medley relay. 

Ledecky, who won four gold and a silver at the Rio Olympics, has now claimed one gold and a silver in Tokyo with the 800m to go and potentially a relay.