Siobhan-Marie O'Connor has retired aged 25 because of "debilitating" struggles with ulcerative colitis.
O'Connor, who took silver at Rio 2016 in the 200m individual medley final, was diagnosed with the condition shortly after she competed at the London 2012 Olympics, aged 16.
The disease left her hospitalised in 2018 and 2019. A serious flare up in her condition after the International Swimming League last November stunted her training.
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And just a month before the Tokyo Olympics is due to get underway, O'Connor has announced her retirement.
"Stepping away from the sport I love has been the hardest decision I've ever had to make," she told BBC Sport.
"I'm very disappointed I haven't been able to compete at the Tokyo Olympics, but I'm so proud of all that I've achieved in my career."
She added: "Having colitis is like having food poisoning every day of your life - you're drained, in pain and have unfortunate symptoms of food poisoning. Sometimes I'd be so tired that I'd end up sleeping for 19 hours of the day.
"Despite all of the setbacks I've always felt it was mind over matter, but my body has been battered and bruised and each flare-up has taken its toll."
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