McClatchey now holds British freestyle records at 100m, 200m and 400m
Double Commonwealth champion Caitlin McClatchey set a British 100m freestyle record of 54.31 seconds in reaching the Olympic trials final in Sheffield.
Swimmers have to achieve the qualifying time set by British Swimming in the heats and finish in the top two in the final to be considered for Beijing.
Fran Halsall also reached the final in 54.88secs, and will now go head-to-head against McClatchey on Tuesday morning.
Jemma Lowe, meanwhile, set a new British record in the 200m butterfly.
Lowe led from start to finish to finish in two minutes 06.64 seconds, knocking more than two seconds off the record held since April 2004 by the now retired Georgina Lee.
It was also a personal best of almost three seconds for the Borough of Stockton swimmer, who claimed the 100m fly record in the same pool last summer.
"I really wasn't expecing that. I just tried to kick my legs as hard as I could," she said.
I am very happy to get the record but didn't think it would last past the next heat
"I was confident I had got the qualification time but I was not expecting that time at all. I just can't believe it."
Ellen Gandy, Jessica Dickons and Hannah Miley also achieved the qualifying time, meaning four swimmers will be chasing two places in the finals on Tuesday morning.
McClatchey and Halsall, meanwhile, are the only two competitors with a chance of qualifying for the Olympics in the 100m freestyle.
Halsall, 17, previously held the 100m freestyle record following her swim in January at the same Ponds Forge pool, where the trials, which double as the British Championships, are being held.
However, Halsall, although clearly comfortable, was more than half a second slower in her heat to leave McClatchey with a clean sweep of British freestyle records in 100m, 200m and 400m.
Scotland's McClatchey, 22, who revealed she had missed some training due to illness, said: "I thought Fran Halsall was going to break it [the record].
"All I wanted was to get the qualifier. I am very happy to get the record but didn't think it would last past the next heat."