James Guy has won Great Britain's second gold of the FINA Aquatics World Championships in Kazan, Russia, by taking victory in the men's 200 metres freestyle.
Guy won silver in the 400m freestyle on Sunday before going one better on Tuesday over four lengths.
The 19-year-old, who trains at Millfield School in Somerset, won in a British record of one minute 45.15 seconds.
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China's Sun Yang was second in 1min 45.20secs and Paul Biedermann of Germany third in 1:45.38, while Ryan Lochte was fourth.
It was another success for Britain after Adam Peaty took gold in the 100m breaststroke on the second day of swimming competition, when Ross Murdoch claimed bronze behind his compatriot and Siobhan-Marie O'Connor clinched bronze in the women's 200m individual medley.
Peaty was in the pool again for the non-Olympic 50m breaststroke and advanced to the final from his semi-final in a world record of 26.42 seconds.
That broke the mark set by Cameron van der Burgh in Tuesday morning's heats of 26.62. The South African is likely Peaty's main rival again in Wednesday's final.
It has been a strong championships for Britain in the pool, with competition running until Sunday.
Guy led after 100m, but trailed both Sun and Lochte with one length to go.
Yet the Briton had enough in reserve to dig deep on the final length and record a memorable triumph over a field stacked with Olympic and world champions.
He broke his own British record, which had stood at 1:45.43, in recording the fastest time in the world this year.
Sun was aiming for a historic 200-400-800 and 1500m freestyle quadruple, but those hopes are now over.