Erika Fairweather’s defence of her world 400m freestyle title was short-lived after the Kiwi swimmer was disqualified for a false start.

The 21-year-old finished fourth in the third heat at the World Aquatics Championships in Singapore on Sunday, with a time of 4min 3.39sec - which would have seen her comfortably qualify in sixth for the final.

Instead, Fairweather was scratched after what the international broadcasters said was a move on the start.

Nine-time Olympic gold medallist Katie Ledecky - the most decorated female swimmer in Olympic history - qualified first with a time of 4:01.04 but lost the highly anticipated battle with Canadian Summer McIntosh.

The 18-year-old launched her packed programme with a dominant performance, romping home in 3min 56.26sec, nearly two seconds clear of China’s Li Bingjie, who was followed by Ledecky in bronze.

It was world record holder McIntosh’s first world title in the event as she pursues five individual golds in Singapore.

Fellow Kiwi Olympian Eve Thomas also missed out on a spot in the final, posting the 14th-fastest heat time with 4:10.10.

Fairweather was out for revenge after narrowly missing out on medalling in the 400m freestyle final at the Paris Olympics last year in what was beforehand dubbed the “Race of the Century”.

She finished 0.26 seconds behind Ledecky, with McIntosh taking second place and Australian Ariarne Titmus - who is not competing in the world championships - taking gold.

“I’m a bit gutted to get fourth, nobody wants to be there,” Fairweather said. “I might be a little bit upset now but I can definitely see the bright side of it already.”

“Being among all those girls is awesome. I’m in one of the most competitive races there is, so I can’t complain.”

Fairweather took out first place in the world championships held before the Olympics in Doha, which none of the three Olympic medallists attended.