Replay - Tancock takes gold
Liam Tancock won Britain's second gold in the pool at the World Championships in Rome and beat his own world record in the final of the 50m backstroke.
Tancock powered home in 24.04 seconds, bettering the mark he set in Saturday's semi-final by 0.04 seconds.
Michael Phelps took his tally for the meet to five golds and one silver as the United States took gold in the men's 4x100m medley relay.
Team GB finished with seven medals in all, their best ever showing.
They won two golds, three silvers and two bronzes in the pool, in addition to diver Tom Daley's gold in the 10m platform and Keri-Anne Payne's gold in the 10km open water.
It improves on their performance at last year's Beijing Olympics, where they won three medals in the pool and three in the open water events.
Forty-three world records were broken at the meet, and some of them may not be beaten for a long time as performance-enhancing, non-textile swimsuits are to be banned from 1 January 2010.
There was British interest in six of the seven finals on the final day, with Hannah Miley finishing fourth in the women's 400m individual medley.
Fran Halsall, who won silver in the 100m freestyle, finished fifth in the final of the 50m free, while David Davies was sixth in the 1500m final.
Loughborough swimmer Tancock, the only British man to win a medal, said of his victory: "It was pretty perfect and I'm pretty pleased with that. I did a world record in the semis, but there were a few things I needed to improve for the final would you believe.
"It is another world record and I'm thrilled to bits I'm world champion after two bronzes in previous world championships. It was a class field and I came out on top this time."
The 24-year-old has now set his sights on success in the 100m backstroke, in which he finished fourth in Rome and which is an Olympic event, unlike the 50m.
Hannah Miley finished fourth in the women's 400m individual medley
"That is my aim, definitely," he said. "This [the 50m] is my bonus event. I just missed out on the 100m [he finished fourth in the final], I'm always practising for that.
"I stepped up my game from the Olympics in Beijing to here and I'm sure I'll do that in the future. I've just got to go back to the drawing board."
Tancock, who finished ahead of Japan's Junya Koga (24.24) and Gerhard Zandberg of South Africa (24.34), now has four world championship medals in total, following his efforts in 2005 and 2007.
Miley, who was sixth in the 200m individual medley, posted a new British record of 4:32.72 and finished behind Katinka Hosszu of Hungary 4:30.31, Kirsty Coventry of Zimbabwe (4:32.12) and Olympic champion Stephanie Rice (4:32.29).
Germany's Britta Steffen added to her 100m crown in the women's 50m freestyle final with a world record time of 23.73, while Halsall, who claimed silver in the 100m, set a new British record of 24.11.
Sweden's Therese Alshammar was second in 23.88, and Australia's Cate Campbell and Dutchwoman Marleen Veldhuis shared the bronze in 23.99.
Davies, who also finished sixth at the Beijing Olympics, clocked 14:57.03. Tunisia's Oussami Mellouli took gold in 14:37.28, Ryan Cochrane of Canada took silver in 14:41.38, and China's Sun Yang took bronze in 14:57.03.
Commonwealth record holder Thomas Haffield finished joint seventh in the final of the men's 400m individual medley.
Ryan Lochte powered to victory, the American finishing in 4:07.01 to beat compatriot Scott Tyler Clary and third-placed Hungarian Laszlo Cseh.
Lochte, who made the most of Michael Phelps' absence, has now won four gold medals at the meet.
The British men's 4x100m medley relay team of Tancock, James Gibson, Michael Rock and Simon Burnett finished eighth in the final, but were later disqualified.
The American quartet of Aaron Peirsol, Eric Shanteau, Phelps and David Walters smashed their own world record by more than two seconds, posting a time of 3:27.28. Germany finished second in 3:28.58, Australia were third in 3:28.64.
Russia's Yuliya Efimova smashed the world record to take gold in the women's 50m breaststroke.
Efimova clocked 30.09 with American Rebecca Soni taking silver and Australian Sarah Katsoulis bronze.