A top White House adviser starkly warned Trump administration officials in late January that the coronavirus crisis could cost the US trillions of dollars and put millions of Americans at risk of illness or death.

The warning, written in a memo by Peter Navarro, President Trump’s trade adviser, is the highest-level alert known to have circulated inside the West Wing as the administration was taking its first substantive steps to confront a crisis that had already consumed China’s leaders and would go on to upend life in Europe and the US.

“The lack of immune protection or an existing cure or vaccine would leave Americans defenceless in the case of a full-blown coronavirus outbreak on US soil,” Navarro’s memo said. “This lack of protection elevates the risk of the coronavirus evolving into a full-blown pandemic, imperiling the lives of millions of Americans.”

Dated January 29, it came during a period when Trump was playing down the risks to the US, and he would later go on to say that no one could have predicted such a devastating outcome.

Navarro said in the memo that the administration faced a choice about how aggressive to be in containing an outbreak, saying the human and economic costs would be relatively low if it turned out to be a problem along the lines of a seasonal flu.

But he went on to emphasise that the “risk of a worst-case pandemic scenario should not be overlooked” given the information coming from China. In one worst-case scenario cited in the memo, more than a half-million Americans could die.

A second memo that Navarro wrote, dated February 23, warned of an “increasing probability of a full-blown Covid-19 pandemic that could infect as many as 100 million Americans, with a loss of life of as many as 1.2 million souls.”

At that time, Trump was still downplaying the threat of the virus. The administration was considering asking Congress for more money to address the situation, and the second memo, which circulated around the West Wing and was obtained by The Times, urged an immediate supplemental spending appropriation from Congress of at least $3 billion.

“This is NOT a time for penny-pinching or horse trading on the Hill,” Navarro wrote in the second memo, which was unsigned but which officials attributed to him. It was unclear whether Mr. Trump saw the second memo, whose contents were first reported by Axios.

The second memo seemed aimed at members of the White House Task Force established by Trump to manage the crisis, and reflected deep divisions within the administration about how to proceed and persistent feuding between Navarro and many other top officials about his role and his views.

“Any member of the Task Force who wants to be cautious about appropriating funds for a crisis that could inflict trillions of dollars in economic damage and take millions of lives has come to the wrong administration.” .