Britain's Rebecca Adlington took a historic gold in the women's 400m freestyle, while her team-mate Jo Jackson claimed the bronze medal.
Adlington timed her late surge to perfection as she became Britain's first female Olympic swimming champion in 48 years.
The 19-year-old from Mansfield pipped Katie Hoff of the United States by just seven 100ths of a second.
Jackson also staged a late rally to edge Coralie Balmy into fourth place.
The British pair both paced their races superbly, reaching 100m and 200m in sixth and eighth place respectively.
Just over half a second separated the field at the halfway point, with Adlington reaching 300m in fifth and Jackson just ahead in fourth.
Hoff looked to have a built a decisive lead and was half a length ahead with 10m to go, but Adlington powered up the final 50m to win it at the wall and become the first British woman to claim gold since Anita Lonsbrough won the 200m breaststroke in 1960.
She left the Italian world record holder Federica Pellegrini and France's world champion Laure Manaudou in her wake, in fifth and eighth.
Adlington and Jackson are also the first British women to win medals in the Olympic pool since Sarah Hardcastle's silver (400m freestyle) and bronze (800m freestyle) in Los Angeles in 1984.
Jackson's third place in a time of 4:03.52 meant Team GB eclipsed their entire medal haul in the pool from Athens 2004 in one race.
In Greece, Britain won two bronze medals, with Stephen Parry (200m butterfly) and David Davies (1500m freestyle) both making the podium.
A thrilled Adlington, who finished in 4:03.22, said she was disappointed with her time but added: "We are both so happy to have two British girls on the podium. We just tried to ignore the times and race the final.
"I don't think either of us are natural sprinters, we don't do the 100m free so we are naturally suited to the back half of the race.
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"I couldn't see what there was on the other side of the pool, I saw Katie and just tried to catch her."
Adlington added: "I don't think either of us expected it and especially a gold and a bronze, it's absolutely amazing.
"It hasn't sunk in yet. I'm just over the moon."
At the Olympics four years ago, where Britain's 30 medals was the country's best tally (aside from the boycotted 1984 Games) since Paris in 1924, the squad had to wait until day seven for their first gold.
It is day three in Beijing and Team GB already have two golds, with Adlington building on cyclist Nicole Cooke's thrilling win in the women's road race on Sunday.
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