Former New York Mayor Michael Bloomberg will not run as a third-party candidate for US president.
He had recently considered running as an independent in November’s election.
Mr Bloomberg said he was concerned about the unconventional and insurgent campaigns of Republican Donald Trump and Democrat Bernie Sanders.
The owner of the Bloomberg news organisation said his candidacy would risk letting in Mr Trump or his fellow Republican Ted Cruz.
He set out his reasoning in an article on his website on Monday entitled: “The risk I will not take“.
Mr Bloomberg said that his run would split the vote. If there was no clear winner in the popular vote, it would create a situation where Congress would choose the president, he said.
“As the race stands now, with Republicans in charge of both Houses, there is a good chance that my candidacy could lead to the election of Donald Trump or Senator Ted Cruz,” Mr Bloomberg wrote.
“That is not a risk I can take in good conscience.”
Analysis: Anthony Zurcher, BBC North America reporter
In the end media tycoon Michael Bloomberg had to acknowledge what seemed obvious to many political observers. If he launched an independent presidential bid, he would be likely to hand the White House to someone like Donald Trump or Texas Senator Ted Cruz.
The New Yorker was overly optimistic about his ability to win enough states to throw a deadlocked election to the House of Representatives, but even if he did, outperforming almost every other third-party candidate in modern American history, the outcome would have been the same – a Republican president. Given the two leading candidates’ extreme positions on immigration, he wrote, that was just too great a risk for him to take.
Mr Bloomberg’s decision can also be read as a tacit acceptance that Hillary Clinton is going to be the Democratic presidential pick. News that he was considering a bid first surfaced as the former secretary of state stumbled in early nominating contests. Now, however, she seems to be pulling away from Bernie Sanders, whose democratic socialism runs counter to Mr Bloomberg’s big-business centrism.
In his editorial, Mr Bloomberg criticised the Republican candidates’ positions on immigrants and Muslims.
“[Trump] has run the most divisive and demagogic presidential campaign I can remember, preying on people’s prejudices and fears,” Mr Bloomberg wrote.
Mr Trump, the Republican front-runner, has pledged to deport nearly 11 million undocumented immigrant workers and called for a ban on Muslims entering the US.
Mr Bloomberg said Mr Cruz held many of the same extreme positions but with less “bombastic” rhetoric.
He has long sought the White House and saw a potential opening as Democratic front-runner Hillary Clinton struggled against Mr Sanders in the early races.
Mrs Clinton has since recovered after a series of wins in large southern states.
Formerly a Republican, Mr Bloomberg holds pro-business positions typical of the party. However, he has broken with Republicans on social issues such as gun rights and gay marriage.
He served as New York mayor for three terms from 2002 to 2013. He focused on boosting the business community and improving transport infrastructure.