Former New York mayor Michael Bloomberg has said he would not mount an independent bid for the US presidency because he feared it would increase the chances that Republicans Donald Trump or Ted Cruz could end up in the White House.

A billionaire media mogul who, while mayor, combined business-friendly fiscal policies with liberal views on gun control and other social issues, Mr Bloomberg could have potentially appealed to centrist voters in a year when candidates from the far left and right of the political spectrum have gained traction.

But Mr Bloomberg said he had concluded that any candidate would be unlikely to win a clear majority in a three-person race.

That would throw the election into the Republican-controlled House of Representatives, which would be able to hand the White House to Mr Trump, a real-estate billionaire, or Mr Cruz, a conservative senator from Texas.

"That is not a risk I can take in good conscience," he said.

Mr Bloomberg never received much interest from American voters.

About 12% of likely voters said they would support him in a three-way race for president with Democrat Hillary Clinton and Mr Trump, according to a Reuters/Ipsos national poll conducted from Wednesday until yesterday.

Among respondents, 41% said they would support Ms Clinton and 31% would support Mr Trump.

The poll of 1,695 likely voters had a credibility interval of three percentage points.

Mr Bloomberg said Donald Trump, who is leading the battle to win the Republican nomination for the 8 November election, had backed policies that would undermine religious tolerance and threaten national security.

Mr Trump has called for building a wall on the US border with Mexico, deporting the country's illegal immigrants and temporarily barring Muslims from entering the country.

"He has run the most divisive and demagogic presidential campaign I can remember, preying on people's prejudices and fears," Mr Bloomberg wrote of Mr Trump.

He said Mr Cruz, a favorite of evangelicals and the conservative Tea Party movement, was divisive as well.

Mr Bloomberg also hit out at Ms Clinton and her rival for the Democratic nomination, Senator Bernie Sanders from Vermont, for criticising free trade and the financial industry.

"Extremism is on the march, and unless we stop it, our problems at home and abroad will grow worse," he wrote.

Spokespeople for Mr Trump and Mr Cruz did not immediately respond to requests for comment about Mr Bloomberg's criticism.

Mr Bloomberg founded and is majority owner of Bloomberg LP, a news and financial information provider that competes with Thomson Reuters Corp.

The fear of a general election contest between Mr Trump and Mr Sanders, a democratic socialist, had driven Mr Bloomberg to begin seriously exploring an independent run, a senior adviser said on condition of anonymity.

But with Ms Clinton pulling away from Mr Sanders in the Democratic race, Mr Bloomberg concluded the path to victory and the rationale for running were gone, the aide said.

Ms Clinton reacted to the news with polite praise, saying she had the "greatest respect" for Mr Bloomberg.

"He has to make his own decisions, but I look forward to continuing to work with him," she said.

Mr Sanders, when asked about Mr Bloomberg's decision not to run, said election laws should be changed to make it easier for people who are not rich, or not friendly with rich people, to run for office.

"I think it's a bad idea for American democracy that the only people who feel in many ways they can run for president are people who have so much money," he said.