Donald Trump had a positive Covid-19 test, quickly followed by a negative, shortly before he went into a presidential debate with challenger Joe Biden last year, according to a new book.

Despite getting the all-clear in that second test, Trump would get so sick with Covid-19 within a week that he had to be urgently hospitalized.

The testing detail emerged Wednesday in a report by The Guardian, which obtained an advance copy of "The Chief's Chief," a memoir by Trump's chief of staff at the time, Mark Meadows.

Trump tested positive three days before his first debate against Biden on September 29, 2020, and exhibited mild symptoms of what was taken to be a cold, the upcoming book recounts.

According to Meadows' account, the result came through just as Trump was leaving the White House for a campaign rally in Pennsylvania on September 26. 

This followed a packed celebration at the White House for newly confirmed Supreme Court Justice Amy Coney Barrett -- a party where VIPs mingled maskless and that subsequently was identified as a super spreader event.

"Stop the president from leaving," Meadows quotes the then-White House doctor as saying after the first test came out, only to be told it was too late.

Meadows wrote that when he informed Trump aboard Air Force One of the positive test, the Republican responded with a phrase rhyming "with 'Oh spit, you've gotta be trucking lidding me.'"

A second test, that Meadows said was a more accurate kind, gave a negative result shortly afterward.

Trump took this as "full permission" to continue his schedule, including that night's rally and the Biden debate three days later.

Debate organizers said the candidates arrived too late to be tested but they affirmed they had done their own tests. Trump's campaign did not, however, inform the debate or Biden about the positive test.

In the book, Meadows writes that he allowed Trump to continue with his schedule after the second, negative test, because "I didn't want to alarm the public."

In a statement Wednesday, former president Trump said the episode did not occur.

"The story of me having COVID prior to, or during, the first debate is Fake News. In fact, a test revealed that I did not have COVID prior to the debate," he said.

Within a week of the rally, Trump was in the hospital.

On October 2, Trump announced on Twitter he and his wife Melania had tested positive for Covid-19. 

He was flown to the presidential facility at Walter Reed National Military Medical Hospital by helicopter from the White House and received intensive treatment for three nights.

A month later, Biden defeated him in the presidential election after focusing his campaign on a promise to mount a more responsible attack on the pandemic.