Image caption Mr Trump said he was not able to leave the private sector and that business remained his passion

Tycoon Donald Trump says he will not run for the US presidency in 2012, declaring he is not ready to leave the private sector.

On Monday, he said he could win, but "business is my greatest passion".

In recent months, the real estate mogul and reality television star had loudly questioned whether President Barack Obama was born in the US.

The media attention that gained, and his existing fame, boosted him to the top tier of candidates.

In a statement on Monday, Mr Trump said that through his "unofficial" campaigning, he had come to realise that "running for public office cannot be done half-heartedly".

Mr Trump, who hosts the reality television programme Celebrity Apprentice, made his announcement on Monday as the NBC television network rolled out its line-up of forthcoming shows.

The lunatic fringe will not be represented in the Republican race. I am, of course, talking about his hair, not his politics Mark Mardell, BBC North America editor Read Mark's thoughts in full

Prior to the announcement, NBC said it would bring back the programme with a different host if Mr Trump ran for president.

The New York businessman had been flirting with a presidential bid for months, even travelling to early primary states like New Hampshire.

Last month, Mr Trump demanded that Mr Obama release copies of his birth certificate, adding fuel to rumours among certain groups that the president was not born in the US.

The president ultimately released the certificate, and Mr Trump took credit for forcing Mr Obama's hand.

Had he run, Mr Trump would have had to make significant disclosures about his financial interests and undergo scrutiny of his business activities, and analysts had suggested that prospect could dissuade a candidacy.

And Mr Trump's tumultuous personal life - played out in past decades across the pages of New York's tabloid newspapers - his ideological inconsistency, his reputation as a showman and his lack of political experience could have proven problematic for serious-minded and doctrinaire Republican primary voters, correspondents say.

Mr Trump's news comes only days after Mike Huckabee, a Republican and former governor of the US state of Arkansas, announced he would not seek the Republican nomination for the presidency.

Former Republican House Speaker Newt Gingrich and Texas Congressman Ron Paul both announced their intention to run last week.

Other potential Republican 2012 candidates include former Massachusetts Governor Mitt Romney, former Utah Governor and US ambassador to China Jon Huntsman, and former Minnesota Governor Tim Pawlenty.

Of those, only Mr Romney and Mr Pawlenty have taken the first official step towards a candidacy, by forming exploratory committees to test the waters and begin raising money.