Former Utah governor Jon Huntsman is dropping his US presidential bid and will endorse frontrunner Mitt Romney for the Republican nomination on Monday, his campaign said.

The timing of the news was a shock, coming hours after Huntsman picked up a major newspaper endorsement in South Carolina, which votes Saturday in a crucial contest that could all but seal the nomination for Romney.

Huntsman staff told ABC News and CNN that the official announcement would come on Monday when the outgoing candidate formally endorses Romney, a fellow Mormon who like him is seen as a moderate Republican.

Campaign spokesman Tim Miller said Huntsman, who was President Barack Obama's envoy in Beijing before entering the contest, was "proud of the race that he ran" but "did not want to stand in the way" of Romney.

Huntsman, who has scheduled a press conference for 11:00 am (1600 GMT) Monday in Myrtle Beach, South Carolina, struggled largely due to his perceived closeness to Obama.

Despite impressive foreign policy credentials and a dramatic campaign launch in June at the Statue of Liberty, his campaign failed to catch on with Republican primary voters.

Huntsman decided not to compete in the first state to vote, Iowa, placing all his bets on New Hampshire, where he felt his more moderate brand of conservatism would get a better hearing.

"They pick corn in Iowa. They actually pick presidents here in New Hampshire," the 51-year-old scion of one of America's richest families opined at one point.

His gamble -- that if he could somehow win or come a strong second in New Hampshire he would revive his lackluster bid -- failed as he ended in third behind runaway victor Romney and Ron Paul, a veteran Texas congressman and small government champion.

In accepting his third-place finish in New Hampshire, Huntsman put on a brave face, sounding triumphant as the confetti rained down and his supporters cheered.

"Ladies and gentlemen, I think we're in the hunt," he said, adding in comments likely to haunt him now: "I'd say third place is a ticket to ride."

Huntsman's popularity had spiked in the polls on the eve of the New Hampshire primary, raising hope among his supporters that his bold strategy was paying off at the final moment.

His late surge came after he struck back hard in a debate at Romney's charge that his service as Obama's first envoy to Beijing should disqualify him from being the party's standard-bearer.

"I will always put my country first," said Huntsman, who made "Country First" his new slogan and vowed first to restore both the US economy and the US public's failed trust in its political elites.

The eventual third-place finish in New Hampshire was, however, the writing on the wall as he headed south into more conservative territory with polls in South Carolina showing him trailing at the bottom of the pack.

Not even the endorsement on Sunday morning of South Carolina's largest paper, The State, appears to have made any difference for a candidate who always seemed more popular in the media than in the voting booths.

On paper he should have shone, but the conservative governor of a well-run state and two-time ambassador with business experience may have misjudged a Republican electorate eager more for red-meat attacks on Obama than his own steady-as-she-goes approach.

"I'm not gonna light my hair on fire," he said in October. "I don't think you have to be crazy to be in the Republican Party."

And in a much-noticed tweet in August, Huntsman broke with the Republican base, saying: "I believe in evolution and trust scientists on global warming. Call me crazy."

His departure leaves a five-way race between Romney, Paul, staunch conservative flag-bearer Rick Santorum, former House speaker Newt Gingrich and Texas Governor Rick Perry. Those five will duel it out later Monday in the next televised Republican debate, in Myrtle Beach.