Boris Johnson is unfit to be prime minister after presiding over a chaotic and incompetent pandemic response that caused many thousands of unnecessary deaths, his former chief aide Dominic Cummings claimed in an excoriating attack.

In a seven-hour hearing before MPs in Westminster, Cummings gave a damning account of the government’s approach, laying much of the blame on Johnson and the health secretary, Matt Hancock.

The ousted aide said the prime minister had failed to grasp the gravity of the situation and held out against lockdowns meaning “tens of thousands of people died who didn’t need to die”. He portrayed Johnson as obsessed with the media and making constant U-turns “like a shopping trolley smashing from one side of the aisle to the other”.

Asked whether the prime minister was a fit and proper person to lead the country through the pandemic, Cummings replied simply: “No.” Apologising for what he said were his own failings, he added: “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.”

Other allegations in Cummings’ no-holds-barred testimony included:

*  Hancock lied repeatedly to colleagues, causing the cabinet secretary – and Cummings – to urge Johnson to sack him, though the prime minister was told that “he’s the person you fire when an inquiry comes along”.

*  Cummings heard Johnson say he would rather see “bodies pile high” than impose a third lockdown – something the prime minister has denied in the House of Commons.

*  The government was woefully under-prepared for the pandemic, with no sense of urgency or plan for steps to protect vulnerable people, such as shielding.

*  Ministers were assured patients leaving hospital for care homes would be tested first but belatedly discovered this was happening “partially and sporadically”.

*  The prime minister’s fiancee, Carrie Symonds, meddled in hiring decisions to try to secure jobs for her friends in a way that was “unethical” and “illegal”.

While Cummings is widely viewed as bitter about his treatment at the hands of his former boss, he is also one of the first key figures from inside No 10 at the height of the pandemic to give public evidence.

In an appearance that spurred calls for a public inquiry to be expedited, the former aide described chaotic scenes in Downing Street in the early days of the pandemic, saying it was “surreal” and comparing it to the alien invasion film Independence Day.

He said that in January and February 2020, as news of the pandemic emerged from China, ministers and senior officials fell victim to what he described as “literally a classic historical example of group-think in action”.

He claimed that only in mid-March was an initial plan to pursue “herd immunity”, by allowing the virus to spread but delaying the peak of the outbreak, belatedly abandoned. Herd immunity “was the whole logic of all the discussions in January and February and early March”, Cummings told the hearing.

He said the prime minister had repeatedly played down the seriousness of the disease, calling it a “scare story”. Cummings even claimed officials deliberately kept Johnson out of emergency Cobra meetings lest he hamper the response to the virus.

“Certainly, the view of various officials inside No 10 was if we have the PM chair Cobra meetings, and he just tells everyone ‘don’t worry about it, I’m going to get [England’s chief medical officer] Chris Whitty to inject me live on TV with coronavirus, so everyone realises it’s nothing to be frightened of,’ that would not help, actually, serious planning.”

No 10 rejected many of Cummings’ claims, including the idea the government had pursued a herd immunity strategy and that border policies were too lax.

The government had sought to undermine the reliability of Cummings’ account before it began. Grant Shapps, the transport secretary, told Sky News: “It’s easy to be the professor of hindsight. For certain, there are things we could have done differently … but I’ll leave it to others to determine how reliable a witness he is.”

Cummings conceded that his account of his lockdown-breaking trip to Durham at a press conference in the Downing Street rose garden had been a “disaster”.

Explaining the decision not to order a lockdown in September, Cummings claimed, as had been previously reported, that Johnson argued at that time Covid was “only killing 80-year-olds”. The Labour leader, Keir Starmer, gave the prime minister the opportunity to deny making that remark at prime minister’s questions, but Johnson did not do so. Starmer accused the prime minister of “chaos, confusion and deadly misjudgment”.

Starmer said the revelations in the hearing had underlined the need for the public inquiry into the pandemic to start work before the spring 2022 date set by the prime minister. “No more delays. A public inquiry needs to start this summer,” Starmer said.

Hancock is expected to be confronted with some of Cummings’ allegations when he answers an urgent question from the shadow health secretary, Jon Ashworth, in the House of Commons on Thursday and later presents a Downing Street press conference.

A spokesperson for Hancock said: “At all times throughout this pandemic the secretary of state … and everyone in DHSC has worked incredibly hard in unprecedented circumstances to protect the NHS and save lives. We absolutely reject Mr Cummings’ claims about the health secretary.”