Sir Brian Leveson, sitting with two other judges, ruled that she had been tried fairly at the Old Bailey last year, and media coverage had not prejudiced the hearing.
Onasanya, a trained solicitor who represented herself, told the court she was innocent, despite having been found guilty and sentenced to three months imprisonment. She served just 28 days in prison.
Sir Brian expressed surprise she had come to court without any legal papers or notes before describing her claim against conviction as “misconceived”.
“In conclusion, this applicant was tried fairly by a jury, who rejected her evidence on oath,” he ruled.
“There was no error of law in the approach of the judge, whose directions were clear and accurate, nor was there any other irregularity with the trial.
“It is a tragedy that she has damaged, probably irreparably, a promising political career, but there is absolutely no basis for challenging her conviction.”