Swimmer Siobhan-Marie O'Connor says she is "absolutely heartbroken" after being ruled out of next week's British trials for the Tokyo Olympic Games.
O'Connor, 25, who won silver for Great Britain at Rio 2016, was diagnosed with ulcerative-colitis in 2012 and has suffered a flare-up of the condition.
In a statement, O'Connor said she had been unable to train for a "consistent and sustained period" since late 2020.
"Unfortunately things have not gone the way that I had hoped." she said.
O'Connor, who was second in the 200m individual medley final in Rio, was diagnosed with the condition shortly after competing at the London 2012 Games.
She was hospitalised in late 2018 and feared she may have to quit the sport, but returned to training the following year and competed in the 2020 International Swimming League (ISL).
Despite her best efforts to continue her training schedule, O'Connor said the "unpredictable" nature of the condition means competing in the British Swimming Championships would be "very difficult."
She added: "I'm absolutely heartbroken but so grateful for the support of my close family and friends during what has been a really tough time.
"I would like to take some time to get well and decide what my next steps will be."
Ulcerative colitis is an inflammatory bowel disease which affects the lining of the large intestine and can result in bouts of extreme fatigue and a weakened immune system.
British Swimming has revised its selection policy for the Tokyo Olympics because of the Covid-19 pandemic, which means there could be further opportunities for O'Connor to attain a qualifying time in the next two months.
In a recent interview with BBC Sport, O'Connor said she hoped sharing her own experience would bring better awareness of the condition.
"Ulcerative colitis and Crohn's are invisible illnesses and that's part of the problem," she said. "You can look really fine on the outside, but be really poorly inside.
"Speaking out has given me strength and I know now that if I'm having a massive battle with chronic fatigue or the pain is really severe that it's OK to tell people and not bottle it up."
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