World Championship finalists Ellen Gandy and Jemma Lowe and British record-holder Fran Halsall all negotiated the heats of the 100 metres butterfly on the first morning of the Olympic trials at the Aquatics Centre.
With only two slots available on the Great Britain team, one world-class swimmer is set to miss out.
As a test event overseas swimmers are allowed to compete and world record-holder Saraj Sjoestroem was quickest in 58.28 seconds, with Gandy heading the Britons into tonight's semi-finals in 58.71secs.
Halsall had missed out in this event at last year's World Championships when Lowe pipped her to the second berth by one hundredth of a second.
Both Gandy and Lowe reached the final in Shanghai, finishing fifth and eighth respectively.
Halsall was the second-fastest Briton in 59.01, while Lowe was 12th through in 59.89.
Joe Roebuck was the fastest Briton through to tonight's final of the 400m individual medley in 4:17.78, two hundredths of a second ahead of world finalist Rob Pavoni.
Hannah Miley was highly impressive as she qualified fastest for tonight's 400m individual medley.
The Scot, second at the World Championships in Shanghai last summer, touched in four minutes 39.49 seconds.
Also through were Aimee Wilmott and Keri-Anne Payne, who is looking to add pool selection to the open water spot she claimed last summer.
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