Sawt Falasteen - Ledecky lays down marker ahead of 400m showdown

The three will go head to head later in the day in one of the most eagerly anticipated races of the championships in the southern Japanese city of Fukuoka.

Defending champion Ledecky of the US clocked the morning's fastest time with 4min, 00.80sec, ahead of Australia's Olympic champion Titmus on 4:01.39.

Canada's McIntosh, the world record holder, had the third-fastest time with 4:01.72.

The three have not gone head to head since the final of the Tokyo Olympics two years ago.

McIntosh broke Titmus's world record in March this year, clocking 3:56.08.

France's Leon Marchand geared up to take a crack at Michael Phelps's long-standing 400m individual medley world record by qualifying second in the heats.

Phelps's benchmark time of 4min, 03.84sec has stood since the 2008 Beijing Olympics but Marchand swam the second-fastest time ever at last year's world championships in Budapest.

Marchand clocked 4:10.88 in the heats to claim the second-best time behind American Carson Foster on 4:09.83, and ahead of Japan's Daiya Seto on 4:10.89.

Marchand, who is coached by Phelps's former mentor Bob Bowman, said he thought he could "do better tonight".

"It was cool -- I couldn't wait to get started," said the 21-year-old.

"The course was a bit long at the end. I did a good first 200 and that's what I wanted to do."

Australian rising star Sam Short will attempt to claim his first individual world championship medal in the men's 400m freestyle.

The 19-year-old had the fastest time of the heats in 3min, 42.44sec, finishing ahead of Austria's Felix Auboeck on 3:44.14 and Brazil's Guilherme Costa on 3:44.17.

Tunisia's Olympic champion Ahmed Hafnaoui was fourth-fastest on 3:44.34.

In the women's 200m individual medley heats, American Kate Douglass came out on top in a time of 2min, 09.17sec, ahead of Australia's Kaylee McKeown on 2:09.50 and Canada's Mary-Sophie Harvey on 2:09.65.

China's Qin Haiyang was fastest in the men's 100m breaststroke heats in a time of 58.26sec.

Olympic and world silver medallist Arno Kamminga of the Netherlands was second-fastest in 58.71, ahead of Lucas Matzerath of Germany on 58.74.

Britain's world record-holder Adam Peaty is not competing in Fukuoka, as he takes a mental health break after falling into a "self-destructive spiral".

In the men's 50m butterfly heats, France's Maxime Grousset clocked the fastest time in 22.74sec.

Britain's Jacob Thomas Taylor Peters followed on 22.85, ahead of Dylan Carter of Trinidad and Tobago on 22.89.

Caeleb Dressel, who won last year's title in Budapest before pulling out of the competition citing unspecified medical concerns, is not competing in Fukuoka after failing to qualify for the American team.

In the women's 100m butterfly heats, China's Zhang Yufei swam fastest with a time of 56.89sec, ahead of Australia's Emma McKeon on 57.05 and Germany's Angelina Kohler on 57.23.

Americans Gretchen Walsh and defending champion Torri Huske, and Canada's Olympic champion Maggie MacNeil all qualified for the semi-finals.

H.Nasr--SF-PST