The world-record holder in the event, Ledecky led from the start to finish in 15:37.34, almost two seconds off her Olympic record in the heats.

Ledecky's teammate Erica Sullivan was not in contention for a podium finish at the halfway mark but surged over the latter half to overtake the field and claim silver in 15:41.41.

While Germany's Sarah Köhler moved through the field after the first 800 m, she could not hold off the late charge from Sullivan, taking bronze one and a half seconds back in a German record of 15:42.91.

Up to two swimmers per National Olympic Committee (NOC) can automatically qualify by swimming that time at an approved qualification event.

Up to one swimmer per NOC meeting that time is eligible for selection, allocated by world ranking until the maximum quota for all swimming events is reached.