Emma Eaves, 12, broke the Southern Counties Championship record for the 100m butterfly at Crystal Palace.
She was one of four members from Brighton Swimming Club to qualify for the National Championships which start on July 27 in Sheffield.
Eaves set a time of 1:08:74 to break the record, before capping a superb championship by finishing within the qualifying time in the 200m butterfly.
Isobel Rogers, 13, will also compete in the 100m butterfly at the Nationals after adding another qualifying time to the one she set in the 200m at Aldershot two weeks ago.
Hannah Belameh, 12, and Jack Smith, 14, also finished within qualfying times and will both be competing in four disciplines.
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