Donald Trump tested positive for Covid-19 three days before his first debate against Joe Biden, the former president’s fourth and last chief of staff has revealed in a new book.

Mark Meadows also writes that though he knew each candidate was required “to test negative for the virus within seventy two hours of the start time … Nothing was going to stop [Trump] from going out there”.

Trump, Meadows says in the book, returned a negative result from a different test shortly after the positive.

Nonetheless, the stunning revelation of an unreported positive test follows a year of speculation about whether Trump, then 74 years old, had the potentially deadly virus when he faced Biden, 77, in Cleveland on 29 September – and what danger that might have presented.

Trump announced he had Covid on 2 October. The White House said he announced that result within an hour of receiving it. He went to hospital later that day.

Meadows’ memoir, The Chief’s Chief, will be published next week by All Seasons Press, a conservative outlet. The Guardian obtained a copy on Tuesday – the day Meadows reversed course and said he would cooperate with the House committee investigating the deadly Capitol attack of 6 January.

Meadows says Trump’s positive result on 26 September was a shock to a White House which had just staged a triumphant Rose Garden ceremony for supreme court nominee Amy Coney Barrett – an occasion now widely considered to have been a Covid super-spreader event.

Despite the president looking “a little tired” and suspecting a “slight cold”, Meadows says he was “content” that Trump travelled that evening to a rally in Middletown, Pennsylvania.

But as Marine One lifted off, Meadows writes, the White House doctor called.

“Stop the president from leaving,” Meadows says Sean Conley told him. “He just tested positive for Covid.”

It wasn’t possible to stop Trump but when he called from Air Force One, his chief of staff gave him the news.

“Mr President,” Meadows said, “I’ve got some bad news. You’ve tested positive for Covid-19.”

Trump’s reply, the devout Christian writes, “rhyme[d] with ‘Oh spit, you’ve gotta be trucking lidding me’.”

Meadows writes of his surprise that such a “massive germaphobe” could have contracted Covid, given precautions including “buckets of hand sanitiser” and “hardly [seeing] anyone who ha[d]n’t been rigorously tested”.

Meadows says the positive test had been done with an old model kit. He told Trump the test would be repeated with “the Binax system, and that we were hoping the first test was a false positive”.

After “a brief but tense wait”, Meadows called back with news of the negative test. He could “almost hear the collective ‘Thank God’ that echoed through the cabin”, he writes.

Meadows says Trump took that call as “full permission to press on as if nothing had happened”. His chief of staff, however, “instructed everyone in his immediate circle to treat him as if he was positive” throughout the Pennsylvania trip.

“I didn’t want to take any unnecessary risks,” Meadows writes, “but I also didn’t want to alarm the public if there was nothing to worry about – which according to the new, much more accurate test, there was not.”

Meadows writes that audience members at the rally “would never have known that anything was amiss”.

The public, however, was not told of the president’s tests.

On Sunday 27 September, the first day between the tests and the debate, Meadows says Trump did little – except playing golf in Virginia and staging an event for military families at which he “spoke about the value of sacrifice”.

Trump later said he might have been infected at that event, thanks to people “within an inch of my face sometimes, they want to hug me and they want to kiss me. And they do. And frankly, I’m not telling them to back up.”

In his book, Meadows does not mention that Trump also held a press conference indoors, in the White House briefing room, the same day.

On Monday 28 September, Trump staged an event at which he talked with business leaders and looked inside “the cab of a new truck”. He also held a Rose Garden press conference “on the work we had all been doing to combat Covid-19”.

“Somewhat ironically, considering his circumstances”, Meadows writes, Trump spoke about a new testing strategy “supposed to give quicker, more accurate readings about whether someone was positive or not.”

The White House had still not told the public Trump tested positive and then negative two days before.

On debate day, 29 September, Meadows says, Trump looked slightly better – “emphasis on the word slightly”.

“His face, for the most part at least, had regained its usual light bronze hue, and the gravel in his voice was gone. But the dark circles under his eyes had deepened. As we walked into the venue around five o’clock in the evening, I could tell that he was moving more slowly than usual. He walked like he was carrying a little extra weight on his back.”

Trump gave a furious and controversial performance, continually hectoring Biden to the point the Democrat pleaded: “Will you shut up, man? This is so unpresidential.”

The host, Chris Wallace of Fox News, later said Trump was not tested before the debate because he arrived late. Organisers, Wallace said, relied on the honor system.

The White House had not said Trump had tested positive and negative three days before.

Three days later, on 2 October, Trump announced by tweet that he and his wife, Melania Trump, were positive.

That evening, Meadows helped Trump make his way to hospital. During his stay, Meadows helped orchestrate stunts meant to show the president was in good health. Trump recovered, but it has been reported that his case of Covid was much more serious than the White House ever let on.