Ellen Gandy broke the British record in the 100 metres butterfly as she booked her place in the Olympics at the trials at the Aquatics Centre.
Gandy made up ground on Fran Halsall in the final metres to touch in 57.25 seconds, cutting 0.15secs off Halsall's 2010 mark.
Halsall was second in 57.56, with both women's times inside that required by British Swimming to qualify for the GB team.
Jemma Lowe, who reached both butterfly finals at last year's World Championships, was third in a final where a world-class athlete was destined to miss out.
Gandy last year became the first British woman to win a global butterfly medal when she was second in the 200m at the World Championships in Shanghai.
All three were part of the Smart Track programme, a group of girls aged 12-14 identified by former national performance director Bill Sweetenham along with John Atkinson and taken to intensive training centres in remote areas of the world.
All three swimmers made their Olympic debuts in Beijing four years ago with Gandy the baby of the GB team.
Just four years on, the trio have made their mark on the global stage.
Both Gandy and Halsall have claimed world silvers as well as assorted European and Commonwealth medals. Lowe reached both finals at last year's World Championships, a year after taking Commonwealth bronze.
Robbie Renwick heads the field into the 200m freestyle final (1:47.25) with Sophie Allen fastest into the women's 100m breaststroke (1:08.44).
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