"We absolutely reject Mr Cummings' claims about the Health Secretary", the spokesman said.
Cummings said that afterwards, despite almost dying himself of Covid, Johnson regretted the decision and said he should have acted like the mayor in "Jaws", who keeps the beaches open despite a monstrous shark lying in wait.
Mr Cummings also suggested that Health Secretary Matt Hancock should have been sacked for numerous failings, accusing him of "lying" to people. But sacking Mr Hancock could appear to confirm Mr Cummings's criticisms. "As I've said before".
Mr Cummings left his role in Downing Street towards the end of 2020 following a power struggle inside No 10 and has since been highly critical of the government.
He described the government's handling of the pandemic "disastrous".
Johnson has admitted that mistakes were made and that lessons need to be learned, but his ministers say they were working at pace in the biggest public health crisis in a century.
During the seven-hour appearance, Mr Cummings was particularly scathing about Mr Hancock, arguing that he "should have been fired for at least 15 to 20 things including lying to everybody on multiple occasions in meeting after meeting in the Cabinet Room and publicly". "I think we're going to kill thousands of people".
Mr Cummings also attacked Mr Hancock for setting a "stupid " target of offering 100,000 Covid tests a day, saying it "hugely disrupted" efforts to "properly" establish a testing system.
He said Sedwill said Johnson should "go on tv and explain that it's like the old chicken pox parties".
Johnson apologises ‘unreservedly’ for ’71 attack in N. Ireland
John Teggart, whose father was killed at Ballymurphy, said the government's public apology should have come from Johnson.
British Prime Minister Boris Johnson was disastrously slow to impose a lockdown in 2020 because he thought Covid-19 was a scare story and even considered getting injected with coronavirus on live television to show it was benign, his former chief adviser said. I think it's a disaster that I acted too late.
"I regarded him as unfit for the job", he added. Cummings said there was had been no plan for the epidemic, and certainly no plan for a lockdown.
Mr Johnson has denied making the comments.
Mr Johnson hit back at some of his allegations, insisting that the government's priority had always been to "save lives".
"We have at every stage tried to minimize loss of life, to save lives, to protect the [health service] and we have followed the best scientific advice that we can".
"The view of various officials inside Number Ten was, if we have the Prime Minister chairing Cobra meetings and he just tells everyone "It is swine flu, don't worry about it, I'm going to get Chris Whitty to inject me live on TV with coronavirus, so everyone realizes it is nothing to be to be frightened of, ' that would be, that would not help actually serious planning", Cummings said".
He said those on the front line of the pandemic were like "lions" being "led by donkeys".
In a blistering attack on the British state, Dominic Cummings told lawmakers that the government was completely unprepared for the worst public health crisis in decades and that ministers, including the prime minister, were on holiday in February 2020, some skiing.