Billionaire George Soros on Thursday delivered a scathing assessment of Donald Trump, calling the US President-elect a "would-be-dictator" who is "going to fail".

On the eve of Trump's inauguration in Washington, Soros said Trump was "gearing up for a trade war" which would have "a very far reaching effect in Europe and other parts of the world".

The "would-be-dictator... didn't expect to win, he was surprised," the Hungarian-born financier told an audience of business leaders and journalists at a Hotel in Davos where the World Economic Forum is being held.

"I personally have confidence that he's going to fail... because his ideas, that guide him are inherently self-contradictory," said Soros, adding that members of Trump's cabinet are each fighting for different interests.

But he predicted the loss of the US' "positive influence in the world in favour of an open society", which would have "a very far reaching effect in Europe and other parts of the world".

Soros, who was a supporter of Trump's Democratic rival Hillary Clinton during last year's campaign, lost nearly a billion dollars as a consequence of the rally prompted by Trump’s surprise election victory, according to press reports.

But the positive reaction in financial markets would not last long, Soros predicted, because ultimately they do not like uncertainty.

US stocks retreated and the dollar fell against most currencies Thursday in the final session before Trump's inauguration on Friday.

On Brexit and Theresa May, Soros predicted the British Prime Minister's spell in power would not last long and said the UK population were "in denial" about the financial consequences of leaving the European Union.

"It's unlikely that Prime Minister May is actually going to remain in power," he said.

"At the moment people in the UK are in denial. The current economic situation is not as bad as it was predicted, they live in hope, but as the currency depreciates, and inflation will be the driving force, that will lead to declining living standards.

"It's going to take some time but when it does happen, they will realise that they are earning less than before, because wages won't rise as fast as the cost of living."

Source: AFP