The ex-New York City mayor is concerned the current field of candidates is not good enough to beat Donald Trump in the 2020 election, his spokesman says.

The 77-year-old is expected to file paperwork this week for the Democratic presidential primary in Alabama.

A total of 17 candidates are currently vying to take on President Trump.

Former Vice-President Joe Biden, Massachusetts Senator Elizabeth Warren and Vermont Senator Bernie Sanders are the Democratic frontrunners.

Some recent opinion polls have suggested that Ms Warren and Mr Sanders - who are regarded as to the left of Mr Biden - might lose against Republican Mr Trump if either won the party's nomination.

Mr Bloomberg's spokesman said: "We now need to finish the job and ensure that Trump is defeated. But Mike is increasingly concerned that the current field of candidates is not well-positioned to do that."

A New York billionaire as president?

So far this is only a single toe entering the water, but it's a significant one. The presence of the former New York mayor and multi-billionaire would set alight the race for the Democratic Party nomination.

The politician closest to Mr Bloomberg's centrist outlook is Joe Biden - but his campaign appears to be fading. Mr Bloomberg has, I understand, been doing a lot of polling in early voting states like Iowa and New Hampshire.

This isn't the first time that he's eyed a run at the presidency, only to eventually decide against. But in the past he's ruled himself out because he thought Americans wouldn't vote for a billionaire New York businessman. That concern no longer applies.

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