World Aquatics Championships: Ariarne Titmus and Leon Marchand break world records

Leon Marchand beat the previous men's 400m individual medley world record set by Michael Phelps (right) back in 2008

Australia's Ariarne Titmus and Frenchman Leon Marchand both set new swimming world records on the opening day in the pool at the World Aquatics Championships in Fukuoka, Japan.

Titmus, 22, won a high-class women's 400m freestyle final in a time of three minutes 55.38 seconds.

Marchand knocked 1.34secs off Michael Phelps' last remaining world record in the men's 400m individual medley.

The 21-year-old's time of 4:02.50 beat the mark set by Phelps in 2008.

"That was insane," said Marchand, who is set to be one of the faces of the 2024 Paris Olympics.

"That was the most painful thing I ever did. To do it here is just crazy. The best is yet to come."

United States' Carson Foster came home more than four seconds behind Marchand in second place, while Japan's Daiya Seto took bronze.

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Olympic champion Titmus shaved 0.7secs off the previous record in the women's 400m freestyle, set by Canada's Summer McIntosh in March.

Defending world champion Katie Ledecky, 26, claimed silver, while New Zealand's Erika Fairweather, 19, won bronze and McIntosh, 16, finished outside the medals in fourth.

"I was really excited for this race," said Titmus on her return to Japan, having won gold at the 2020 Olympics in Tokyo.

"I haven't really raced the best in the world since the Olympics. Katie and Summer always put up a great fight and I'm glad we could put on a show."

The Australian team of Mollie O'Callaghan, Shayna Jack, Meg Harris and Emma McKeon also broke the world record in the women's 4x100m freestyle relay, winning gold in 3:27.96, ahead of the United States and China.

The British team of Anna Hopkin, Lucy Hope, Abbie Wood and Freya Anderson set a national record in finishing fourth in 3:33.90.

"We're all a little bit gutted just to miss out on the medals, but still really happy to have moved it on as a quartet," said Anderson.

Australia's men also won 4x100m freestyle relay gold, while the British team of Lewis Burras, Matthew Richards, Jacob Whittle and Duncan Scott was disqualified in the heats.

Earlier, Australia's Sam Short, 19, won the opening final of the championships, claiming gold in the men's 400m freestyle after touching home just two hundredths of a second ahead of Tunisia's Olympic champion Ahmed Hafnaoui, 20. Germany's Lukas Martens, 21, won bronze.

And Britain's Jacob Peters, 22, finished second in his men's 50m butterfly semi-final, ahead of compatriot Ben Proud, 28, in fourth, as both qualified for Monday's final.

But James Wilby just missed out on a place in the men's 100m breaststroke final and Katie Shanahan was disqualified in the women's 200m individual medley semi-finals.