Trump was among those who called into question whether President Barack Obama was born in the US [Reuters]

Donald Trump has said he will not run for the US presidency, choosing to stick with hosting his reality TV show over a bid for the Republican nomination for next year's election.

The real estate mogul made his announcement at a Manhattan hotel on Monday as NBC, which airs his show, The Celebrity Apprentice, rolled out its fall lineup.

"I will not be running for president as much as I'd like to,'' Trump said.

Some public opinion polls had once shown Trump leading the slow-to-coalesce Republican field, which still lacks a clear front-runner.

Trump used his star power as a television celebrity to publicly flirt with a campaign, even though it was
never entirely clear whether he really is a Republican.

In the past few months, he delivered speeches to national Republican groups and travelled to states that hold early primary elections.

Trump was the loudest of many voices to call into question whether President Barack Obama was really born in the United States, becoming the unofficial leader of the "birther" movement.

He suffered a major blow when Obama produced a longer version of his birth certificate that was further proof of what most Americans had already decided - that the president was born in Hawaii in 1961.

Dwindling support

Trump took credit for the release even though it robbed his candidacy of its signature issue and the buzz around his possible campaign died down.

But in his announcement on Monday, Trump said he believes he would have won if he had decided to run.

"This decision does not come easily or without regret, especially when my potential candidacy continues to be
validated by ranking at the top of the Republican contenders in polls across the country," he said.

Trump's support for the Republican nomination fell from 26 per cent in April to just 8 per cent in early May in surveys done by Public Policy Polling.

He is the second Republican in a matter of days to say no to a bid for the Republican nomination. Mike Huckabee announced on Saturday that he wouldn't seek the presidency.

Trump had floated the idea of a presidential candidacy in both 1988 and 2000 but claimed he was more serious than ever this time, citing the weak economy and the sense that the United States was in decline.

On Monday his office released a formal statement in which he said: "Ultimately, business is my greatest passion and I am not ready to leave the private sector".