Former Republican presidential nominee Mitt Romney has labelled the party's 2016 White House front runner Donald Trump a "phony" in an astonishing attack.

In a speech at the University of Utah on Thursday, the former Massachusetts Governor urged Republicans to shun the real estate billionaire for the good of the party.

Mr Romney has joined a growing group of anxious Republican figures attempting to slow Mr Trump's march to the nomination.

"Here's what I know," he said, "Donald Trump is a phony, a fraud." "His promises are as worthless as a degree from Trump university.

"He's playing the American public for suckers - he gets a free ride to the White House and all we get is a lousy hat."

Mr Romney - who is not running for the White House this time - also sought to undermine Mr Trump's success story.

"A business genius he is not," he said.

He said a win for Mr Trump at the Republican convention in Cleveland in July would gift Democrat Hillary Clinton the keys to the White House.

Mr Romney said Mr Trump, who has courted controversy with remarks against women, Muslims and Mexicans, "has neither the temperament nor the judgement to be president".