Contestants must avoid politics, be gracious in defeat and never forget to pay homage to Donald Trump, the billionaire property mogul who owns the Miss USA franchise.

But after failing to win the 2012 contest last week, one contender managed to smash all three rules.

Sheena Monnin, 26, renounced her Miss Pennsylvania title, called the competition "trashy" and claimed the show, won by Miss Rhode Island Olivia Culpo, was rigged.

The psychology student posted on Facebook the day after the contest: "I can no longer be affiliated in any way with an organisation I consider to be fraudulent, lacking in morals, inconsistent, and in many ways trashy."

She claimed that Miss Florida saw a list of the five finalists before it was announced, proving, she said, that the show was rigged.

Trump said: "When she's using the word 'fraud', that's pretty strong, so we're going to sue her on that basis."

The show's organisers have released an e-mail she sent to them in which she wrote: "I refuse to be part of a pageant system that has so far and so completely removed itself from its foundational principles as to allow and support natural-born males to compete in it."

They suggested that may have had something to do with her resignation.

Last month, after legal threats from a transgender contestant who wanted to take part in Miss Canada, the organisation announced that transgender contestants would be eligible.

On Friday, Monnin denied this, saying: "I know what I heard and I know what I witnessed."