Jazmin Carlin and Hannah Miley recorded world-leading times in their finals on day two of the British Championships.
Carlin, 22, missed last year's Olympic Games but her comeback 800m freestyle time of eight minutes 18.56 seconds was a near six-second personal best.
Miley, 23, clocked 4:35.67 to win the 400m individual medley in Sheffield ahead of Aimee Wilmott, who also qualified for the World Championships.
Chris Walker-Hebborn beat Liam Tancock in the men's 100m backstroke final.
With Rebecca Adlington retiring earlier this year, British Swimming is searching for an athlete who can replace her medal-winning performances at major events and Carlin's time, in Adlington's favoured event, suggests she could be ready to step up this summer.
"I honestly don't know what to say - it's kind of a surprise," Carlin told BBC Sport.
"To drop six seconds is normally unheard of at this age, but I'm over the moon."
The Swansea-based swimmer, whose Olympic preparations were hampered by glandular fever and tonsillitis, added: "Last year really hurt me and knocked my confidence and took me a while to get over but it's June, a new year, and I'm ready to face the best in the world."
Miley impressed at the short-course World Championships late last year, claiming two medals, but admits she was not entirely confident heading into the 400m individual medley final.
"I had doubts in my my mind because so many things can go wrong," said Miley, who won World Championship silver two years ago.
"I've gone in with a fractured hand but raced the best I could.
"I've looked at results around the world and I knew what I had to do but getting on the team was the main thing - so I can relax now."
Walker-Hebborn's 53.38 - a new personal best and the fourth quickest time recorded by any swimmer in the event this year - was enough to hold off the challenge of Commonwealth champion Tancock, who may now miss out on a fifth World Championship appearance.
"I've been looking up to Liam for many years and it's nice to come out on top for once, but it's bittersweet at the same time because we get on so well," Walker-Hebborn told BBC Sport.
Sophie Allen qualified for the World Championships in the 100m breaststroke after winning in a time of 1:07.70 while Daniel Fogg, who was already assured of places in the 400m freestyle and open water events in Barcelona, secured a third berth with victory in the 800m freestyle.
Elsewhere, it was a story of several near misses.
Olympic finalists Jemma Lowe and Elizabeth Simmonds won their respective 200m butterfly and 200m backstroke finals but failed to achieve the qualification times they needed to reach Barcelona.
There was also disappointment for Joe Roebuck in the 200m butterfly - his time of 1:56.49 was just one one-hundredth of a second outside the required mark.
And there was controversy in the men's 200m breaststroke semi-finals, with Olympic finalist Andrew Willis disqualified for a "faulty stroke".
The Bath University-based 22-year-old had finished clear of Olympic silver medallist Michael Jamieson's 2:11.30 with a time of 2:09.82.
"Apparently it was something to do with my pull-kick-pull on the turn, but I swam it the way that I always do so there's not much I can do about that," Willis told BBC Sport.
His coach Dave McNulty protested against the decision, but the original disqualification was upheld, which means it will now be moved to a jury of appeal.
Should the ruling not be overturned, his elimination is bad news for British Swimming's medal prospects in Barcelona.
In the multi-classification disability races, Paralympic champion Josef Craig clocked a new world record of 4:41.13 in the 400m freestyle.
The 16-year-old, who has cerebral palsy, paid tribute to team-mate Jonathan Fox, who also finished inside the old world record time.
"He's a great friend out of the pool, but when we're in it we have a really big go at one another and it's going to be an amazing battle [at the World Championships in August]," said Craig.
Highlights on the third day of the British Championships include the return of Jamieson for the 200m breaststroke final, while 2011 world 50m backstroke champion Tancock will hope for an improvement on his performance over 100m.