Pershing Square (PSH ) fund manager and outspoken supporter of Donald Trump, Bill Ackman, has warned an ‘economic nuclear winter’ is coming if the US president does not ‘call a time out’ on his tariff plans.

Amid a global market meltdown, billionaire investor Ackman took to X, formerly known as Twitter, late on Sunday to urge Trump to stop his ‘global economic war against the whole world at once’, referring to the imposition of ‘reciprocal’ tariffs announced last week and set to take effect on Wednesday.

These follow the baseline tariff of 10% against all US trading partners that came in force on Saturday. Despite sending stock markets into a tailspin and massively increasing fears about a recession, Trump has remained steadfast so far, stating that ‘sometimes you have to take medicine to fix something’.

While Ackman agreed with Trump on ‘fixing the global system of tariffs that has disadvantaged the country’, he warned that ‘disproportionate tariffs on our friends and our enemies alike’ had started a trade war with the entire world, ‘destroying confidence’ in the US as a trading partner, as a place to do business, and as an investment market.

Ackman said Trump has an opportunity to ‘call a 90-day time out’ that would provide him breathing room to negotiate and resolve ‘unfair asymmetric tariff deals’ in a way that would see trillions of dollars flow into the US.

‘If, on the other hand, on 9 April we launch economic nuclear war on every country in the world, business investment will grind to a halt, consumers will close their wallets and pocket books, and we will severely damage our reputation with the rest of the world that will take years and potentially decades to rehabilitate,’ he said.

Ackman warned Trump that should he bring forward more tariffs, the US was heading for a ‘self-induced, economic nuclear winter, and we should start hunkering down’.

A further barrage of levies will see markets crash, new investment stop, consumers stop spending, and businesses curtail investment and lay off workers, with smaller businesses experiencing the most pain, warned Ackman.

‘Business is a confidence game. The president is losing the confidence of business leaders around the globe,’ he said. 

‘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,’ he said.

Pershing Square, the £8.9bn London-listed fund invested primarily in North American stocks, lost 8.4% on opening this morning but has rallied since – helped by a short-lived rumour that the Trump administration was indeed considering a tariff pause. 

Trading down 2.1% at £33.00 at 3.50pm, PSH is now down 13.5% over three trading sessions. 

Ackman has occasionally employed hedges successfully to mitigate market falls, notably during the Covid pandemic. 

However, Citywire columnist James Carthew speculated today that Ackman’s strident criticism of the tariff policy suggested that was unlikely this time.