Britain's Adam Peaty warned the men's 50m breaststroke world record could fall again in a fierce battle over the sprint distance in Wednesday's final.

The British ace broke his own world record from 2015 by clocking 26.10 seconds in Tuesday's heats, then lowered it again to 25.95sec in the evening's semi-finals.

The record is under threat for a third time at the world aquatics championships in Budapest with two-time gold medallist Cameron van der Burgh and 2015 bronze medallist Kevin Cordes also in the mix.

Depite shattering the world record twice on Tuesday, the 22-year-old Peaty says he can go faster in the final.

"There's always more to come, everyone says you work 10 out of 10, but it's really a nine to take it a step further," said Peaty.

Having failed in his bid to complete 'project 56' over 100m — he swam 57.47sec in Monday's breaststroke final — Peaty succeeded in breaking the 26-second barrier over the shorter distance.

"After the Rio Olympics, everyone was talking about 'project 56', I thought 'go on then'," he said.

"Then this morning it was 'project 26', I just dived in tonight and it felt good, but it's nothing without the gold."

Gold medal machine Katie Ledecky of the USA can win her fourth title at these Budapest championships in the women's 200m freestyle final.

She is the fastest into the final, clocking 1:54.69, with Australia's Emma McKeon just 0.30 behind her.

Ledecky's performance in the semi-final came within minutes of her winning the women's 1500m freestyle final to pick up the 12th world gold of her young career.

She passed Missy Franklin's tally of 11 world golds to become the most decorated female swimmer in the history of the championships and her 13th title beckons over 200m.

Having won four gold medals at the 2013 world championships in Barcelona, Ledecky won five golds two years ago in Kazan.

She could leave Budapest with six more titles, with the 400m, 1500m and 4x100m freestyle golds already in the bag.

Chinese superstar Sun Yang can underline his domination of the 800m freestyle with a fourth straight gold in the final.

The 25-year-old first, already a winner in the 200m and 400m freestyle in Budapest, has claimed the 800m title at each of the past three world championships.

He was fifth fastest in qualifying with Italy's Gregorio Paltrinieri, the 1500m freestyle Olympic champion, the quickest at 7:45.31 in the heats.

South Africa's Chad le Clos says he is bracing himself to race in Budapest's 'Lion's den' atmosphere against home town favourite Laszlo Cseh in the men's 200m butterfly final.

Japan's Daiya Seto, the reigning 400m individual medley world champion, was the fastest in the semi-finals at 1:53.04 with Cseh 0.19sec back and compatriot Tamas Kenderesi third at 0.95.

Le Clos was the fourth fastest at 1.06sec off the pace.

With two Hungarian swimmers in the final, the home crowd will roar their deafening support at the Duna Arena.

"Four or five swimmers could win it, Seto, Chad le Clos or Kenderesi, there are a lot of great swimmers there," said Cseh. "It's great to race here and feel all the people cheering, it helps us a lot."