Proud had set a personal best time in the heats of 48.70 to qualify fastest for the final. That moved him up to third in the British all-time rankings.
But the 20-year-old, who had won the 50m butterfly time trial on Tuesday, could not quite repeat that performance in the final and was edged out for the title by Bath’s former Leander and Plymouth College swimmer Calum Jarvis.
Jarvis won by just 0.04 of a second. The Bath swimmer clocked 48.79 with Proud touching in 48.83.
Neither swimmer was inside the Great Britain World Championship qualifying mark, but they will have a chance of a relay spot and possibly a discretionary place, depending on other results.
Proud will swim again in the 50m freestyle event on Friday. He is British record holder in the one-length race and Commonwealth Games champion.
Proud said afterwards on social media: “A big learning curve for me. I couldn’t back up my performance from the heat and it cost me a win. (Fortunately it went to a well-deserving friend who stepped up when it mattered!).
“Now I can go back and learn from it, so when the time comes to step up for my country I will be able to deliver.”
Meanwhile, Leander’s Joe Hulme enjoyed more success in London. The Plymouth College pupil added to his 100m backstroke junior silver on the opening day with another silver in the 200 IM.
Hulme clocked 2:02.95 in the junior boys (EG) finals. He was just edge out by Doncaster’s Jarvis Parkinson, who finished in 2:02.43.
Former Kelly College pupil Cameron Brodie won the men’s 200m butterfly in 1:57.71.
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