Dominic Cummings has apologised to the families of those who died from coronavirus - and admitted the Government fell "disastrously short" in its handling of the pandemic.
The PM's ex-aide kicked off his much anticipated Commons hearing by admitting mistakes had been made during the crisis.
Mr Cummings told MPs: "The truth is that senior ministers, senior officials, senior advisers like me fell disastrously short of the standards that the public has a right to expect of its Government in a crisis like this.
"When the public needed us most the Government failed.
"I would like to say to all the families of those who died unnecessarily how sorry I am for the mistakes that were made and for my own mistakes at that."
MPs kicked off the hearing by pressing Mr Cummings about the early handling of the pandemic, when he was Boris Johnson's top aide in No10.
He said many institutions failed to grasp the seriousness of the situation in the early months of 2020 and he should have been sounding the alarm earlier.
He said: "When it started, in January, I did think in part of my mind, 'Oh my goodness, is this it? Is this what people have been warning about all this time?'
"However, at the time the PHE (Public Health England) here and the WHO (World Health Organisation) and CDC, generally speaking, organisations across the western world were not ringing great alarm bells about it then.
"I think it is in retrospect completely obvious that many, many institutions failed on this early question."
Mr Cummings tore into his former colleagues, saying the Government "didn't act like it was the most important thing in February, never mind in January".
The Government was "not on a war footing" in February, he said.
Lots of key people "were literally skiing" in February - and Boris Johnson was on holiday, the ex-aide said.
The PM has come under fire over his failure to attend five key COBRA meetings in January and February, with claims that he missed the critical meetings because he was writing a book about Shakespeare.
Mr Cummings admitted the PM had not attended early meetings, but he had sent another aide Ben Warner to go along.
In a damning assertion, Mr Cummings claimed the PM though Covid was a "scare story" and dismissed it as the latest "swine flu"
And he accused Mr Johnson of wanting to get Chief Medical Officer Chris Whitty to inject him with the virus live on TV.
The PM went on to contract coronavirus in April, resulting in a stay in intensive care where he was given oxygen by doctors.
Mr Cummings also blasted Matt Hancock for offering "completely hollow" assurances that pandemic preparations were in place.
He told the committee: "I would like to stress and apologise for the fact that it is true that I did this but I did not follow up on this and push it the way I should've done.
"We were told in No 10 at the time that this is literally top of the risk register, this has been planned and there's been exercises on this over and over again, everyone knows what to do.
"And it's sort of tragic in a way, that someone who wrote so often about running red teams and not trusting things and not digging into things, whilst I was running red teams about lots of other things in government at this time, I didn't do it on this.
"If I had said at the end of January, we're going to take a Saturday and I want all of these documents put on the table and I want it all gone through and I want outside experts to look at it all, then we'd have figured out much, much earlier that all the claims about brilliant preparations and how everything was in order were basically completely hollow, but we didn't figure this out until the back end of February."
