Romanian teenager David Popovici warned his rivals that a landmark 100m-200m freestyle double is “only the beginning” as he triumphed again at the World Championships in Budapest.

American Jim Montgomery is the only other man to win both races at the same Worlds, doing the double in 1973.

Great Britain’s Lewis Burras finished seventh in Popovici’s wake.

The anticipated duel between Popovici and seven-time Olympic champion Caeleb Dressel did not materialise after the American withdrew from the 100m freestyle semi-finals on Tuesday.

Dressel, who spoke of feeling disillusioned with the sport in May, subsequently ended his campaign at the Duna Arena on Wednesday on medical grounds.

“For his long-term health that’s the right decision to make today,” Lindsay Mintenko, the USA team’s swimming managing director, said at the start of the evening session.

“He’s just not fit to compete right now.”

For legal reasons, she could not be more specific on the problem, but added: “Ultimately this is Caeleb’s decision.”

In Dressel’s absence, Popovici revelled in the limelight as he held off Grousset’s challenge in the closing stages.

When asked if he expected this level of success, the teenager said: “I don’t think so, but I was positive throughout.

“It is only the beginning. We have a long, long road ahead of us.”

Canadian 15-year-old sensation Summer McIntosh won the 200m butterfly gold as she took another 0.6 seconds off her personal best to win in two minutes 5.2 seconds.

American Hali Flickinger finished strongly over the final 50m to claim silver ahead of China’s Zhang Yufei.

“As soon as I stepped out with all the energy and excitement from the crowd, I just fed off that. I got a lot of adrenaline and motivation and put it down,” said McIntosh.

Great Britain’s Tom Dean, who won 200m freestyle gold in Tokyo last summer, clocked a new personal best in the 200m individual medley, but was some way off French winner Leon Marchand in fifth.

Britain’s Medi Harris finished seventh in the 50m backstroke as Kylie Masse touched first in a grandstand finish to claim another Canadian victory ahead of American Katharine Berkoff and France’s Analia Pigree.

Britain’s Freya Anderson, who finished fourth in the 200m freestyle final on Tuesday, failed to make the 100m freestyle, coming sixth in her semi-final.

Team-mate Anna Hopkin was faster, but finished ninth quickest and missed out on the final line-up.

However fellow Britons Brodie Williams and Luke Greenbank both made their way into the 200m backstroke final, while Abbie Wood and Molly Renshaw also advanced in the 200m breaststroke.

The evening’s action finished with the United States winning the 4x200m freestyle relay as Katie Ledecky’s stellar third leg allowed Bella Sims to anchor home her team in relative comfort and claim a new championship record.

It also meant Ledecky, 25, became the most successful female swimmer in world championships history, surpassing Natalie Coughlin, with her 21st career medal.

However the championships are over for Australia’s Shayna Jack, who has withdrawn from the event after breaking her hand in a freak training accident.

The 23-year-old was part of Australia’s gold medal-winning 4x100m relay team and was expected to be among the medals in the women’s 100m freestyle.

It had been her first international meet since serving a doping ban.

Jack, who is hoping to recover in time for the Commonwealth Games, said she was “in shock and disbelief”.