Michael Bloomberg has announced he will not be challenging Donald Trump for the US presidency in 2020.

The Jewish billionaire wrote in an editorial published on the Bloomberg website that Mr Trump “is a threat to our country” and that he believed he would defeat him in a head-to-head contest.

But he said: “I am clear-eyed about the difficulty of winning the Democratic nomination in such a crowded field.”

“Many people have urged me to run,” he added. “Some have told me that to win the Democratic nomination, I would need to change my views to match the polls.”

Analysts had suggested his bid for the Democratic nomination would struggle, particularly if former vice president Joe Biden joins the race.

In December Mr Bloomberg flew to Iowa in a visit that was widely considered to be testing the waters for a presidential run.

He previously considered running in 2016 — as an independent — but concluded he could improve Mr Trump’s chances of victory by attracting voters who would otherwise support Hillary Clinton.

In Tuesday’s editorial, Mr Bloomberg urged Democrats to nominate a candidate “who will be in the strongest position to defeat Donald Trump and bring our country back together.

“We cannot allow the primary process to drag the party to an extreme that would diminish our chances in the general election and translate into ‘Four More Years.’”