Michael Bloomberg confirmed Sunday that he is running for president, joining the crowded field of Democrats seeking to take on his fellow New York billionaire, President Donald Trump.

His 11th-hour entry highlights the fluid nature of the Democratic contest just three months before the first primary. “I'm running for president to defeat Donald Trump and rebuild America,” Bloomberg, 77, said on his website as a $30 million Bloomberg ad campaign hit US airwaves.

The announcement ended weeks of speculation that the former New York mayor was preparing for a run at the White House.

He had prepared the groundwork for a possible bid for the Democratic nomination in recent weeks by registering as a candidate in primary voting states and filing paperwork with the Federal Election Commission.

“We cannot afford four more years of President Trump's reckless and unethical actions,” the billionaire businessman said.

“He represents an existential threat to our country and our values. If he wins another term in office, we may never recover from the damage,” Bloomberg added.

With a personal fortune of $50 billion, Bloomberg's candidacy will likely shake up the open race, with 17 candidates already vying to be the Democratic nominee to take on Trump next year.

Former vice president Joe Biden leads the race ahead of left-wingers Elizabeth Warren and Bernie Sanders, with moderate Pete Buttigieg in fourth place, according to national polls.

Bloomberg is poised to run as a centrist and analysts suspect that he could take away some of the support enjoyed by fellow moderate Biden.

Some believe that his “self-made man” image and support for fighting global warming makes him the best challenger for Trump.

Others on the left of the party who favor Warren or Sanders see him as the sort of billionaire they would like to tax heavily to reduce inequality.

“As a candidate, I'll rally a broad and diverse coalition of Americans to win,” Bloomberg said in his statement, adding that he would “stand up” to Trump's “bigotry.”

Bloomberg touted some of his achievements as New York mayor, including getting the city back on its feet after the September 11, 2001 attacks — he was first elected in January of the following year, taking over from Rudy Giuliani — and banning smoking in restaurants and bars.

He said he was best placed to take on Trump on the issues of gun violence and climate change.

Last week he apologised for his longtime support of controversial “stop and frisk” policing policy that disproportionately targeted black and Latino New Yorkers during his time as mayor.