Jenna Talackova, 23, was notified by pageant organizers last week she was disqualified from a pool of 65 contestants after it was learned she had been born male from her own online Univision profile page, the Toronto Star reported.
Condemnation of her denial led to an online petition that had more than 22,000 supporters by Monday night, the Star said.
In 2003, the pageant modified its entry requirements to include only "natural born" females in the competition.
Pageant spokesman Andrew Lopez told the QMI Agency there was no substance to allegations of discrimination as the rules are clear.
"This is the first time we've ever had somebody disqualified for this reason," Lopez said.
The pageant's Web site has a brief statement saying Talakova "did not meet the requirements to compete despite having stated otherwise on her entry form.
"We do, however, respect her goals, determination and wish her the best."
From her home in Vancouver, Talackova told the Star she is consulting a lawyer about her exclusion from the pageant, which will be held May 19.