Boris Johnson was reluctant to tighten Covid restrictions as cases rose last autumn because he thought people dying from it were "essentially all over 80", Dominic Cummings has claimed.
He also said the prime minister had messaged him to say: "I no longer buy all this NHS overwhelmed stuff."
Mr Johnson had wanted to let Covid "wash through the country" rather than destroy the economy, Mr Cummings said.
The claims came in an interview with BBC political editor Laura Kuenssberg.
It is the first time Mr Cummings - Mr Johnson's former chief adviser - has given a one-on-one TV interview during his career in politics.
In response, Downing Street said the prime minister had taken the "necessary action to protect lives and livelihoods, guided by the best scientific advice" throughout the pandemic.
And the government had prevented the NHS "from being overwhelmed through three national lockdowns", a spokesperson added.
In a wide-ranging interview, Mr Cummings also said that, near the start of the pandemic last year, Mr Johnson had wanted to keep his weekly face-to-face meetings with the Queen going - but he had to warn his boss that she might die if she caught coronavirus.
And he defended his controversial decision to drive to his parents' farm in County Durham after the first lockdown started, but admitted he had not "come clean" about all the reasons behind it, including "security concerns" around his family home in London.
This is the first major interview Mr Cummings has given, but he has answered MPs' questions on the government's response to Covid.
The claims made at that session were explosive, but he's since been criticised for failing to provide the evidence to back up some of those assertions.
Dominic Cummings: The Interview will be broadcast in the UK on BBC Two at 19:00 BST on Tuesday, and it will be available on BBC iPlayer and BBC Sounds
Covid cases dipped last summer but began to rise rapidly again as autumn started, prompting a debate within government about what measures were needed.
Mr Cummings told the BBC that he, UK chief scientific adviser Sir Patrick Vallance and England's chief medical officer Professor Chris Whitty had pushed for tougher restrictions from mid-September - Sir Patrick and Professor Whitty declined to comment.
Mr Cummings went on to allege Mr Johnson had said: "No, no no, no, no, I'm not doing it."
The prime minister had "parts of the media and Tory party screaming" not to increase restrictions and "always referred" to the Daily Telegraph, for which he had previously written a column, as "my real boss", Mr Cummings said.
On 13 October, with Covid deaths having risen to more than 100 a day, Labour leader Sir Keir Starmer called for a "circuit-breaker" lockdown of two to three weeks, but the government decided against this.
In a WhatsApp message sent on 15 October, shared with the BBC, Mr Johnson appears to have described himself as "slightly rocked by some of the data on Covid fatalities".
The "median age" for those dying was between 81 and 82 for men and 85 for women, the prime minister allegedly wrote, adding: "That is above life expectancy. So get Covid and Live longer.
"Hardly anyone under 60 goes into hospital... and of those virtually all survive. And I no longer buy all this NHS overwhelmed stuff. Folks I think we may need to recalibrate... There are max 3m in this country aged over 80."
He reportedly went on to write: "It shows we don't go for nationwide lockdown."
But on 31 October the prime minister announced a four-week lockdown for England to begin on 5 November, saying this was needed to protect the NHS as figures suggested deaths could reach "several thousand a day" without "tough action".
Speaking on Tuesday, Business Minister Paul Scully defended the PM's actions during the autumn, adding that economic restrictions also had an impact on people's health and lives and "you have to take all of those factors into account".
He said the 17-day so-called firebreak lockdown in Wales, introduced around two weeks before England's autumn lockdown, "didn't have a particularly big effect at the time".
"These decisions are not quite as black and white as necessarily we can make them now," he told Radio 4's Today programme.
Queen meetings row
Looking back to the start of the pandemic, Mr Cummings told the BBC he had intervened to stop Mr Johnson continuing to see the Queen, then aged 93, for weekly face-to-face meetings.
Mr Cummings claimed that, on 18 March last year, the PM had said: "I'm going to see the Queen... That's what I do every Wednesday. Sod this. I'm going to go and see her."
He added that he had told Mr Johnson: "There's people in this office who are isolating. You might have coronavirus. I might have coronavirus.
"You can't go and see the Queen. What if you go and see her and give the Queen coronavirus? You obviously can't go."
He continued: "I just said, 'If you give her coronavirus and she dies, what are you going to [do]? You can't do that. You can't risk that. That's completely insane.'
"And [the PM] said - he basically just hadn't thought it through - 'Yeah...I can't go'."
Downing Street denied that this incident took place and Buckingham Palace declined to comment.
Long drive north
On 27 March last year, four days after the first lockdown began, Mr Cummings drove his wife and son from London to his parents' farm in County Durham.
While staying there, he made a 30-mile road journey to Barnard Castle on 12 April, which he later said had been to test his eyesight before the 260-mile drive back to London.
When these details were revealed they provoked huge anger - and accusations of double standards at a time when the government had banned all but essential long-distance travel.
In his BBC interview, Mr Cummings said that, during the Barnard Castle trip, he had been trying to work out "Do I feel OK driving?"
He also said he had decided to move his family to County Durham before his wife fell ill with suspected Covid because of security concerns over his home in London.
Asked why he had given a story that was "not the 100% truth" when he held a special press conference in the Downing Street rose garden on 25 May, Mr Cummings admitted that "the way we handled the whole thing was wrong".
"What I should have done is either just resigned and said nothing about anything," he said, "or I should have spoken to my family and said, 'Listen, we're just gonna have to come clean about the whole thing.'"
Mr Cummings was repeatedly challenged throughout the interview to back up his version of events.
He says many of his claims will be corroborated if there is a public inquiry.
Labour's shadow health minister Justin Madders said NHS workers, patients, and the relatives of Covid victims would find Mr Cummings's interview "shocking and difficult to hear".
"The revelations are further evidence that the prime minister has made the wrong calls time and again at the expense of public health," he added.
"Boris Johnson is reckless, unfit to govern and a public inquiry cannot be delayed".
A Downing Street spokesperson said: "Since the start of the pandemic, the prime minister has taken the necessary action to protect lives and livelihoods, guided by the best scientific advice.
"The government he leads has delivered the fastest vaccination rollout in Europe, saved millions of jobs through the furlough scheme and prevented the NHS from being overwhelmed through three national lockdowns.
"The government is entirely focused on emerging cautiously from the pandemic and building back better."
Sir Iain Duncan Smith - who briefly employed Mr Cummings as an adviser when he was Conservative leader, said the prime minister should be judged on the decisions he made, rather than the processes that led up to them.
Of Mr Cummings, he said: "He obviously is a very bitter man, I know I employed him, I sacked him. I know exactly how he works, this will go on for some time."