A billionaire backer of Donald Trump has urged the US president to pause his recently announced trade tariffs, or risk “a self-induced, economic nuclear winter”.

Amid market turmoil, hedge fund manager Bill Ackman said the president should take three months to allow countries to renegotiate their trading relationships with the US.

On Monday, Mr Ackman’s warning was echoed by other prominent Wall Street figures, with JPMorgan Chase chairman Jamie Dimon saying that Trump’s tariffs risked pushing up prices for Americans.

Despite the shockwaves, the American president is moving ahead, with the White House rushing to label a rumour he might put new tariffs on pause as “fake news”.

The rumour on Monday morning that Trump was considering a 90-day pause briefly lifted a swiftly sinking stock market after it was reported on financial network CNBC.

The White House almost immediately shot down the report, showing Trump’s commitment to his new import taxes. Stock prices largely stabilised afterward.

The “baseline” tariffs of 10% on most country’s goods that Trump announced last week have already gone into effect, while the higher “reciprocal” rates he wants to impose on the “worst offenders” are expected later this week. Some countries are seeking to negotiate lower rates with the White House.

The new tariffs, added to steep levies Trump has already put on goods from Canada and Mexico, as well as all automobile imports, are worrying business and economic leaders that they will push up prices for American consumers and spark a global trade war.

The head of BlackRock, the world’s largest asset manager, said on Monday that the tariffs will raise prices and possibly inflation, and contribute to an economic downturn, according to media reports.

“Most CEOs I talk to would say we are probably in a recession right now,” Larry Fink, the firm’s CEO, told a meeting of the Economic Club of New York.

Goldman Sachs on Sunday said there is a 45% probability of the US entering a recession within the year, after estimating a 35% probability a week ago, before Trump unveiled his tariff plans at an event called “Liberation Day”.

Trump says the import taxes will boost his country with new jobs and investment.

He defended them on Sunday, telling reporters aboard Air Force One that “sometimes you have to take medicine to fix something”.

In his post on X on Sunday, Mr Ackman acknowledged the Trump argument that the global trade system had “disadvantaged” the US.

But, he wrote, tariffs that Trump had imposed were “massive and disproportionate”, and did not distinguish between American friends and enemies.

Mr Ackman, the billionaire founder of Pershing Square hedge fund management company, became a high-profile supporter of Trump, a Republican, in July 2024.

He had previously backed the rival Democratic Party, and his intervention was seen as an important electoral endorsement from the world of business.

The “reciprocal” rates from the Trump administration, which can reach up to 50%, will be levied on some important manufacturing centres in Asia.

Numerous countries have vowed to respond, and China has already retaliated with new tariffs of its own on goods imported from the US. Trump on Monday threatened to put an additional 50% tariff on goods from the country, which would bring the total taxes he intends to charge to more than 100%.

Trump had launched an “economic war against the whole world at once” that risked shattering investor confidence in the US, Mr Ackman commented.

Mr Ackman said the American leader now had “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”.

His post on Sunday indicated that he felt the ball was back in Trump’s court – after an earlier message on X which urged leaders of other countries to “pick up the phone” to make a deal with Trump.

As stock markets around the world continued their slump on Monday, the head of banking giant JPMorgan Chase offered his own take, warning of “many uncertainties” around the new tariffs policy.

In a letter to shareholders, Mr Dimon said the tariffs will “likely increase inflation and are causing many to consider a greater probability of a recession”.

“The quicker this issue is resolved, the better because some of the negative effects increase cumulatively over time and would be hard to reverse,” he wrote.

Trump’s officials have downplayed the recession risk. The baseline 10% tariff is already in effect, with the higher rates faced by some countries due to come into effect on Wednesday.

Speaking aboard the presidential plane on a flight back to Washington DC on Sunday, Trump himself said European and Asian countries were “dying to make a deal”.