Katie Ledecky smashed her own world record by almost two seconds to storm to gold in the women's 400 metre freestyle final at Rio 2016.

The 19-year-old American, who set her previous marker in 2014, led from the off as she left her opponents in her wake, touching the wall in three minutes and 56.46 seconds.

Great Britain's Jazz Carlin was almost five seconds slower in taking the silver medal, with Leah Smith of the United States finishing third.

"The 3:58 this morning [in the heats] felt really easy and I knew if I could just push the back half really hard I could make it happen tonight," Ledecky told NBC after the race.

Ledecky has now won all 13 individual finals she has entered at the Olympics and World Championships and will look to continue her unbeaten streak on Tuesday in the finals of the 200m freestyle, an event in which she holds the second-fastest time in the world in this year (1:54.43) behind Sweden's Sarah Sjostrom.

If Ledecky can pull off golds in the 200m and 800m freestyle in Rio, she will become the second woman ever (Debbie Meyer, USA in 1968) to achieve that treble.