After belittling precautionary measures against the the ongoing coronavirus pandemic, US President Donald Trump has tested positive for the virulent disease just 32 days before the November 3 election that will determine his future, throwing his campaign and the world into a vortex of uncertainty.

Taking to Twitter late Thursday night, Trump said: "Tonight, @FLOTUS and I tested positive for Covid-19. We will begin our quarantine and recovery process immediately. We will get through this together."

Also in a tweet shortly, the First Lady said: "As too many Americans have done this year, @potus & I are quarantining at home after testing positive for Covid-19. We are feeling good and I have postponed all upcoming engagements.

"Please be sure you are staying safe and we will all get through this together."

The Covid-19 infection is a personal and political blow to Trump who had shown disdain for many of the precautions prescribed by health authorities to fight the pandemic.

It could shake the confidence of his hardcore supporters who had followed his example by not wearing masks and questioning the health codes.

Following the development, the White House sent out a revised press schedule listing only "a phone call on Covid-19 support to vulnerable seniors" for Friday, adding that several events have been cancelled, including a fundraiser in Washington and a campaign rally in Florida.

His personal physician Sean Conley's statement said: "The President and First Lady are both well at this time and they plan to remain at home within the White House during their convalescence.

"The White House medical team and I will maintain a vigilant watch, and I appreciate the support provided by some of our country's greatest medical professionals and institutions.

"Rest assured I expect the President to continue carrying out his duties without disruption while recovering, and I will keep you updated on any future developments."

Neither Trump nor the White House has said if he will withdraw from official duties as President while under treatment.

If he does, Vice President Mike Pence will have to officiate as President.

Trump is 74 and overweight at about 122 kg, and the two conditions put him at a greater risk for the ravages of the disease.

If he is asymptomatic, he may be able to function under great restrictions, but if he has to be hospitalised, then he may have to hand off power to Pence depending on the severity of the disease.

That would also throw off the entire campaign.

Even a 14-day quarantine, if he is asymptomatic, would eat up almost half the time left in the campaign.

Under the US Constitution, Speaker of the House of Representatives Nancy Pelosi, a Democratic, is next in line of succession after Pence.

Earlier on Thursday night, Trump had tweeted that he and his wife were quarantining themselves pending the outcome of the tests after one of his closest White House adviser, Hope Hicks, who had travelled with him on the Air Force One presidential jet on Wednesday, had tested positive for the coronavirus.

Hinting at how Hicks may have been infected, he told Fox News: "She has a hard time, when soldiers and law enforcement comes up to her, you know, she wants to treat them great, not say, 'Stay away, I can't get near you'. It's a very, very tough disease."

An NBC News political reporter, Monica Alba, had earlier tweeted a picture of Hicks leaving Air Force One in Cleveland, where she had travelled with Trump for Tuesday's presidential debate, and said she was not wearing a mask.

With his macho bluster, Trump showed disdain for wearing masks and other Covid-19 precautionary measures.

He always appeared in public without a mask and most officials and others appearing with him also dispensed with it.

Many of his supporters appeared at his tightly packed rallies without masks.

During Tuesday's debate. Trump mocked his Democratic Party challenger Joe Biden for scrupulously wearing masks.

"I don't wear masks like him. Every time you see him, he's got a mask. He could be speaking 200 feet away from them and he shows up with the biggest mask I've ever seen," the President said.

Trump said earlier that he had taken the hydroxychloroquine (HCQ) as a preventive measure, especially around the time his valet came down with Covid-19, but discontinued it.

Funding the economic recovery and allocating relief money for individuals, states and cities, and businesses hangs in the balance while the Democrats and the Republicans bicker.

The House of Representatives controlled by the Democrats passed a $2.2 trillion Covid-19 relief package on Thursday, which has been opposed by the Republicans and is unlikely to pass the Senate.

But Trump has generally kept away from the negotiations with the Democrats leaving it to Treasury Secretary Steven Mnuchin.

In the first financial reaction to the uncertainty, the Dow Jones futures plunged by about two per cent, while in early trading in Europe the British stock market index, FTSE, was down nearly three quarterAper cent and the German DAX about a quarter per cent.

Trump has had close brushes with Covid-19 infection in the past.

Besides his personal valet who tested positive for the virus in May, others around him who have been infected include National Security Adviser Robert O'Brien, his son Donald Jr's girlfriend Kimberly Guilfoyle and many Secret Service employees.

Pence's Spokesperson Katie Miller tested positive in May, but he did not quarantine himself.

Hicks, who had worked for the Trump Organisation, was one of the key officials in Trump's 2016 campaign and joined the White House a communications director.

She quit the job in 2018 and returned in March as counsellor to the president.

Trump's Twitter announcement was trolled by scores of people who posted harsh attacks on him, many pointing out the "karma" of his scepticism about Covid-19 precautions coming back to haunt at a crucial time in the campaign..