Former US Republican presidential nominee Mitt Romney will not run for President in 2016.

Three weeks after unexpectedly saying he was considering a third campaign for the White House, the former Massachusetts governor told members of his staff he is out of the race.

Mr Romney jumped back into the presidential discussion earlier this month when he told a small group of former donors in New York that he was eyeing another White House run.

But he has discovered that several of his past supporters and major fundraisers had defected to former Florida governor Jeb Bush.

The former governor of Massachusetts had jumped back into the presidential discussion on January 10, when he surprised a small group of former donors at a meeting in New York by telling them he was eyeing a third run for the White House.

It was a monumental change for Mr Romney, who since losing the 2012 election to President Barack Obama had repeatedly told all who asked that his career in politics was over and that he would not again run for president.

In the days since that meeting in New York, which caught several in attendance off-guard, Mr Romney made calls to former fundraisers, staff and supporters, and gave three public speeches in which he outlined his potential vision for another campaign.

"I'm thinking about how I can help the country," he told hundreds of students on Wednesday night at Mississippi State University.

In that speech, and what amounted to a campaign stop a few hours before at a restaurant, Mr Romney sounded every bit like a politician preparing to run for president.

"We need to restore opportunity, particularly for the middle class," Mr Romney said.

"You deserve a job that can repay all you've spent and borrowed to go to college."

President Obama is barred from a third term, and Hillary Clinton is the presumed Democratic frontrunner. The Republican race remains wide open.

The exit of Mr Romney from the campaign most immediately benefits the other favourites of the party's establishment wing - including Jeb Bush, New Jersey Gov Chris Christie, Florida Sen Marco Rubio and Wisconsin Gov Scott Walker.

The more conservative side of the field is largely unchanged - with a group of candidates that will likely include Kentucky Sen Rand Paul, Texas Sen Ted Cruz and former Arkansas Gov Mike Huckabee.

Mr Romney's aides had acknowledged a third campaign would have been more difficult than his second, but insisted he would have had the necessary financial support, noting his supporters raised more than 1 billion US dollars during the 2012 election.

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