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".

Hedge fund manager Bill Ackman, who lent his support to Trump before the 2024 presidential election, said the president should take three months to allow countries to renegotiate their trading relationships with the US.

Share prices in Europe and Asia have continued to plummet on Monday, as markets react to the sweeping, global tariffs that Trump announced last week.

Despite the shockwaves, the American president has defended his new import taxes, saying "sometimes you have to take medicine to fix something".

He says the move will boost his country with new jobs and investment, but economists warn that prices could rise for Americans and spark a trade war.

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.

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

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.

In his announcements last week, Trump announced a 10% "baseline" tariff on imports to the US, with higher rates of up to 50% faced by dozens of other countries - including a number of 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 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 continue 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".