Natalie du Toit says she has no idea why she was disqualified for "unsporting behaviour" in the South African 5km Open-Water Championship race in Durban at the weekend.
The amputee star, who touched first in the women's race, appealed the decision.
But an official said the appeal had been unanimously rejected by a six-member committee.
Du Toit effectively swam her nearest rival, Michelle Webber, out of space in the final sprint for the touch pad.
She explained that she had intentionally swum in her opponent's slip, and each time Webber had moved to her right, she had followed her to stay in the slip.
"It's allowed," said Du Toit, who yesterday started competing in the pool events to seal her selection for the Paralympics.
"The men's race at the Beijing Olympics [in 2008] was won like that. I didn't hit her, I didn't scratch her. It's been done to me all the time in international races.
"I walked away from that race thinking I can actually swim internationally now," added Du Toit, who competed at the 2008 Olympics.
She will attempt to qualify in Portugal in June for the women's 10km open-water race at the London Olympics.
"I don't understand it. At the end of the day I don't know what I did," she said.
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