The 34-year-old was part of the men's eight who won gold in Rio de Janeiro in 2016 and also a bronze in London four years earlier.
"My time as a rower is up. After 20 years and two Olympic medals I have come to the realisation I have nothing more to give and nothing left to gain," he told the BBC.
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"The rescheduling of the Tokyo Olympics to 2021 gave finality to my decision."
Ransley said he had hoped to be in the eight again in Tokyo, where he intended to end his career, but doubts had already begun to niggle before the postponement due to the COVID-19 pandemic.
The years of early starts, of three training sessions a day and a "selfish and monastic existence" to test his body to the limits had taken its toll and the fire was burning out.
"A friend suggested that if you stop loving something it is impossible to give it your all, and I believe that she is right," he said.
"I have used up everything I had, and I know that to get myself in the necessary condition to compete for a seat in 2021 is a step too far." (Reporting by Alan Baldwin, editing by Toby Davis)
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