Australian swimming star Mack Horton has signalled that his feud with China's Sun Yang is as bitter as ever, refusing to share a podium with Sun after the 400m final at the world championships.

Key points: Mack Horton called Sun Yang a "drug cheat" after beating him at the 2016 Rio Olympics

Mack Horton called Sun Yang a "drug cheat" after beating him at the 2016 Rio Olympics Sun was handed a three-month doping suspension back in 2014

Sun was handed a three-month doping suspension back in 2014 Sun is currently facing allegations of doping violations that could result in a ban from the 2020 Tokyo Olympics

Horton finished second as Sun claimed a record fourth-consecutive 400-metres freestyle final in South Korea but when the time came to stand on the dais for the medal presentation, Horton stood behind it.

The Australian also refused to pose for photos with his rival, who was suspended for doping in 2014, but happily posed with Italian bronze medallist Gabriele Detti.

Asked what his overriding emotion was, Horton replied: "Frustration. I think you know in what respect."

Horton (left) called Sun Yang a drug cheat at the 2016 Rio Olympics. ( AP: Lee Jin-man )

"I don't think I need to say anything.

"His actions and how it has been handled speaks louder than anything I could say."

Sun is currently facing allegations of doping rule violations that could result in a ban from the 2020 Tokyo Olympics and he has requested a public trial at the Court of Arbitration for Sport in September to defend himself.

The World Anti-Doping Agency is challenging a decision by FINA, swimming's world governing body, merely to warn him over incidents during a doping control team's attempts to take blood and urine samples at his home in China last September, while allowing him to continue competing.

Mack Horton (left) stood behind the podium rather than share it with Sun Yang (centre). ( AP: Mark Schiefelbein )

Sun said: "I am aware of the rumours that have been going around, but I try to just concentrate on my swimming and I will keep trying to put a lot of effort in my swimming."

The Chinese world champion said he was aware Horton has a problem with him.

"Disrespecting me was OK, but disrespecting China was unfortunate," Sun said.

"I feel sorry about that."

Sun's presence at the world meet has drawn the ire of some swimmers, including Horton, who is the only swimmer to beat Sun in the past eight years, taking gold in the 400m freestyle at the 2016 Rio Olympics.

Horton only posed for photos with bronze medallist Gabriele Detti. ( Reuters: Kim Hong-Ji )

After that race, Horton called Sun a "drug cheat" for his three-month doping suspension in 2014.

Horton's refusal to take the podium caught the eye of some of his former and current teammates.

"Absolutely awesome to see Mack Horton protesting clean sport by not getting up on the podium next to Sun Yang #cleansport," tweeted David McKeon, who swam the 4x200m relay with Horton in Rio.

Five-time Olympic medallist Melanie Wright wrote: "Congratulations Mack Horton — Olympic and now World champion; shame you won't get the gold medal for it #cleansport."

Gold for Titmus and women's 4x100m relay stars

The reignition of the Horton-Sun beef somewhat overshadowed a spectacular win by Australian Ariarne Titmus in the women's 400m final, upsetting previously unbeatable American Katie Ledecky.

Ariarne Titmus (right) clawed back Katie Ledecky's (left) big lead to win comfortably in the 400m freestyle. ( AP: Mark Schiefelbein )

Eighteen-year-old Titmus led through the first 200m — dipping under world-record pace on the first lap — before Ledecky moved in front with 250 metres to go.

The American was still ahead by 0.62 seconds turning for home, but Australia's Commonwealth Games champion pulled even midway through the last lap and surged to the wall first, beating Ledecky by 1.21 seconds.

Ledecky won her first 400m world title as a 16-year-old in 2013 and has since won 400m gold at the 2015 and 2017 world championships (just two of her 14 world title wins) as well as 800m gold at London 2012 and four gold medals at Rio 2016.

Australia's women earned further bragging rights over the US with a win in the 4x100m freestyle final by just 0.81 of a second.

Australia and the US have won the 4x100m freestyle relay at all the major meets for years. ( AP: Lee Jin-man )

The team of Bronte Campbell, Brianna Throssell, Emma McKeon and Cate Campbell continued the battle for ascendancy with the Americans that can be traced back to the 2012 Olympic Games in London, when Australia claimed gold.

One year later, the US just pipped the Campbells, McKeon and Alicia Coutts to the 2013 world title, then Campbell-led teams won the 2015 world title and 2016 Olympic gold, before the US took gold at the 2017 world champs.

Australia's wins at the 2018 Pan-Pacific championships and now another world championship sets up a tantalising clash at next year's Olympics in Tokyo.

The Australian men, for their part, won bronze behind the US and Russia in their relay.

AP/ABC