Simon Case said he had "never seen a bunch of people less well-equipped to run a country", according to text messages shared with the Covid Inquiry.
In an exchange of private messages sent before he became Cabinet Secretary in July 2020, Mr Case described Downing Street staff as "mad" and "poisonous".
He told his precedessor Sir Mark Sedwill that many “top-drawer people” he had asked to replace No 10 official Tom Shinner “refused to come because of the toxic reputation of his operation”.
Mr Case was appointed to the role in September 2020 by former Prime Minister Boris Johnson.
The disclosure came hours after it was revealed that Mr Johnson had considered injecting himself with Covid-19 on TV to show it did not pose a threat.
Lord Edward Udny-Lister became the latest top official to appear before Lady Hallett’s inquiry this week.
In his written statement to the inquiry, Mr Johnson’s former chief of staff said that in the early pandemic the then-prime minister had offered to be injected with the disease live on television to “demonstrate to the public that it did not pose a threat”.
“It was an unfortunate comment” at a time “when Covid was not seen as being the serious disease it subsequently became”, Lord Udny-Lister told the hearing.
Much of Tuesday’s evidence centred on Mr Johnson’s reluctance later that year to impose another lockdown.
Lord Udny-Lister said in his written statement that he recalled the then-premier saying in September 2020 that he would rather “let the bodies pile high”.
“Whilst this was an unfortunate turn of phrase, it should be borne in mind that by this point the Government was trying to avoid a further lockdown given the already severe impact on the economy and education,” the former top aide said.
Mr Johnson also referred to “whisky and a revolver” during a meeting with officials in October 2020, and spoke about “medieval measures” to tackle the pandemic, according to notes of the then-chief scientist Sir Patrick Vallance shared with the inquiry.
Sir Patrick also complained that Mr Johnson and his then-chancellor Rishi Sunak were “clutching at straws”.
Questioned about allegations of an unhappy staff culture in Downing Street, Lord Udny-Lister largely blamed his predecessor as Mr Johnson’s chief of staff, Dominic Cummings, saying “he was not an easy man to deal with” who always believed his “was the right view”.
He added: “I think that it would have been better if the prime minister had dealt with it perhaps when the opportunity occurred to him certainly in the July of 2020, if not earlier.
“There was a personality clash that was constantly going on.”
People should have been treated “more respectfully”, Lord Udny-Lister said, describing some of the WhatsApp exchanges shown to the inquiry as “pretty appalling”.
Earlier, Simon Ridley, the former head of the Cabinet Office Covid-19 taskforce, faced questions about the role and influence of the official body over the course of 2020.
In his notebook, detailing a meeting in early October, Sir Patrick wrote: “Very bad meeting in no.10… PM talks of medieval measures than ones being suggested.
“Perhaps we should look at another approach and apply different values… Surely this just sweeps through in waves like other natural phenomena and there is nothing we can do.
“As Simon Ridley said final slide, PM said ‘Whisky and a revolver’. He was all over the place. CX (Chancellor) using increasingly specific and spurious arguments against closing hospitality. Both of them clutching at straws…
“There are really only three choices for the high prevalence areas… 1) Do a proper lockdown 2) Use military to enforce the rules 3) Do nothing and do a ‘Barrington Declaration’ and count the bodies (poor, old and BAME). When will they decide.”
In another extract from the notebooks, dated October 25, Sir Patrick suggested that Mr Johnson was often “buffeted” in his decision-making by Mr Sunak.
Sir Patrick wrote: “Ridley meeting – positioned PM meeting as ‘a chance to step back/but avoid making a whole load of decisions that then get undone by Cx’. I asked what PM thinks objectives are ‘what he wants to achieve is a series of mutually incompatible options’. He ‘owns’ the reality for a day and then is buffeted by a discussion with Cx.”
Earlier, Mr Ridley acknowledged that officials working on the Cabinet Office Covid taskforce were “blindsided” by Mr Sunak’s Eat Out to Help Out scheme.
He admitted there was some surprise when the then-chancellor’s plan to encourage people to get back out to restaurants in summer 2020 was first presented.
The inquiry has already heard criticism of the policy from, among others, Sir Chris Whitty, who privately referred to it as “eat out to help out the virus”.
Mr Ridley was asked by lead counsel to the inquiry Hugo Keith KC if he was “extraordinarily concerned” that such a major policy was not brought before the taskforce.
“Things happen that surprise. We were focused on the advice we could give in the context of the steps of the May 2020 document.
“This was announced as government policy. I didn’t spend time worrying particularly about the whys and wherefores of that,” he said.
Put to him by Mr Keith that this was because he was “blindsided by the Treasury and there was nothing you could do”, he said: “Correct.”