Matt Richards and Tom Dean won gold and silver in the 200m freestyle to secure Great Britain’s first medals at the World Aquatic Championships in Japan.
Richards, 20, produced a remarkable finish to beat Olympic champion Dean in one minute 44.30 seconds.
South Korea’s Hwang Sun-woo took bronze while 2022 world champion Romania’s David Popovici finished fourth.
“I’m very tired, but over the moon. It was a packed field,” Richards said.
“It’s amazing to come away with GB 1-2 again.”
The result follows Britain’s gold and silver at the Tokyo Olympics, when Dean, now 23, and Duncan Scott finished first and second.
Popovici, 18, led from the start in Tuesday’s final and looked set to retain his title but faded badly in the home straight as the Brits closed.
Former world champion Karen Pickering, commentating on BBC Radio 5 Sports Extra, said: “It was spectacular. Those two guys swam the race perfectly.
“They sat on other swimmers’ shoulders, they dug deep and their 20 meter closing was absolute perfection.”
Another BBC swimming expert, Steve Parry, added: “Popovic, the Romanian superstar, called the future of the sport, the reigning world champion and the British boys have knocked him down. What a swim from Richards to become world champion.
“GB needed it, but whoever dreamed of the first and second? It will give the team confidence.”
Britain failed to win a medal over the first two days in Fukuoka, with the women’s 4x100m freestyle relay team coming fourth on Sunday and Jacob Peters and Ben Proud finishing fourth and fifth respectively in the men’s 50m butterfly final on Monday.
Richards was part of the British men’s 4x100m freestyle quartet which was disqualified from the Games on the opening day.
Eight-time world champion Adam Peaty is not competing after taking a break from the sport for mental health reasons.