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A billionaire hedge fund boss who backed Donald Trump has turned on the president warning his tariffs risk an “economic nuclear winter” .

Bill Ackman , a prominent financier and one-time Trump supporter, said the tariffs were a “mistake” and called for a 90-day “time-out” to allow more time to negotiate.

Mr Trump’s trade war has sent the global economy reeling in recent days, with sharp falls in indices overnight in Asia and on Monday morning in the UK and Europe.

He warned Trump’s trade war will hammer small businesses disproportionately and destroy confidence in the US as a trading partner.

“We are heading for a self-induced, economic nuclear winter, and we should start hunkering down,” he said on X.

“By placing massive and disproportionate tariffs on our friends and our enemies alike and thereby launching a global economic war against the whole world at once, we are in the process of destroying confidence in our country as a trading partner, as a place to do business, and as a market to invest capital.

“The president has an opportunity to call a 90-day time out, negotiate and resolve unfair asymmetric tariff deals, and induce trillions of dollars of new investment in our country.”

Mr Ackman leads top New York hedge fund Pershing Square, one of the world’s largest investment groups. He endorsed Mr Trump for president in last year’s election but denied he did so to win a job in the new administration.

On Monday, he said the US was “100pc behind” Mr Trump’s attempt to fix a system of tariffs that “disadvantaged” America, but warned falling stock markets would force employers to cut back on investments and consumers to reign in spending.

Mr Ackman said that if the US launched an “economic nuclear war on every country in the world” then business investment will “grind to a halt”.

“What CEO and what board of directors will be comfortable making large, long-term, economic commitments in our country in the middle of an economic nuclear war? I don’t know of one who will do so,” he said.

He added: “And it is not just the big companies that will suffer. Small and medium size businesses and entrepreneurs will experience much greater pain. Almost no business can pass through an overnight massive increase in costs to their customers. And that’s true even if they have no debt, and, unfortunately, there is a massive amount of leverage in the system.

“Business is a confidence game. The president is losing the confidence of business leaders around the globe. The consequences for our country and the millions of our citizens who have supported the president – in particular low-income consumers who are already under a huge amount of economic stress – are going to be severely negative. This is not what we voted for.”

A former donor to the Democratic party, Mr Ackman has called himself a “Bill Clinton democrat” in the past.

However, last year he said he no longer wanted to be associated with the Democrats because of anti-Semitism on Ivy League campuses and “racist” diversity initiatives.

Mr Ackman first endorsed Mr Trump’s campaign last July, having previously been a long-time supporter of the Democratic Party.

At the time, Mr Ackman said he believed Mr Trump’s presidency would help drive up the value of American stocks.

“I am an investor who manages funds that own some of the best, principally American, businesses in the world. In a better governed and managed America, these business [sic] will do better and increase in value faster,” he said in a tweet last October.

Top hedge fund managers including Daniel Loeb and Ken Griffin later shared similar views in suggesting Mr Trump’s presidency would give a boost to US stock markets.

In his third quarter letter to investors, Mr Loeb, the founder of New York hedge fund Third Point, said he expected Mr Trump’s “America First” would “unleash productivity and a wave of corporate activity” that would boost US stock markets.

Speaking at the DealBook summit last December, Mr Griffin said he believed Mr Trump’s presidency would be an opportunity to end the “regulatory and litigation-induced paralysis” of the Biden era and “bring America back to a nation of principles, of strength, of prosperity and possibility”.

Mr Trump later appointed hedge fund billionaire Scott Bessent as his treasury secretary. Mr Bessent started his career at George Soros’s hedge fund in 1990 before starting his own multibillion-dollar hedge fund, Key Square Group, in 2015.

The Trump administration now appears to be losing the support of its billionaire backers amid the market slump.

Stanley Druckenmiller, the billionaire founder of Duquesne Capital, late on Sunday made a rare post on social media platform X, formerly Twitter, suggesting he opposed Mr Trump’s trade policies.

“I do not support tariffs exceeding 10% which I made abundantly clear in the interview you cite,” Mr Druckenmiller said, in response to a tweet that highlighted an interview in which he had previously said “the risks [of tariffs] are overblown relative to the rewards.”

Jamie Dimon, the chief executive of JP Morgan, on Monday also called for Trump’s tariffs to be overturned, as he warned the levies threaten to drive up inflation.

“There are many uncertainties surrounding the new tariff policy,” Mr Dimon said in his annual letter to JP Morgan’s shareholders.

“The quicker this issue is resolved, the better because some of the negative effects increase cumulatively over time and would be hard to reverse. In the short run, I see this as one large additional straw on the camel’s back.”

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