Dominic Cummings said he called for Matt Hancock to be sacked “almost every day” due to alleged “criminal” behaviour but claimed Boris Johnson was advised to retain the health secretary because “he’s the person you fire when an inquiry comes along”.

The prime minister’s former top adviser levelled incendiary allegations at the health secretary, including that he held back tests and lied to the public and fellow ministers. No 10 said the prime minister retained “full confidence” in Hancock.

In testimony on Wednesday, Cummings said officials were “terribly let down by senior leadership” and it was like “lions led by donkeys”.

He singled out Hancock, saying he should have been fired for “at least 15 to 20 things – including lying to everybody on multiple occasions”. He said he had suggested this to the prime minister, as did the then cabinet secretary, Mark Sedwill.

Asked whether that meant some decision-makers should be worried about facing corporate manslaughter charges, Cummings said there was “no doubt at all that many senior people performed far, far disastrously below the standards which the country expects” – and that Hancock was “one of those people”.

When challenged by the Conservative MP Greg Clark, the chair of the science and technology committee, to provide evidence of wrongdoing, Cummings said there were “numerous” examples.

On Wednesday evening, Hancock was asked for his reaction to Cummings’ allegations. The health secretary told reporters: “I haven’t seen this performance today in full, and instead I’ve been dealing with getting the vaccination rollout going, especially to over-30s, and saving lives. I’ll be giving a statement to the House of Commons tomorrow and I’ll have more to say then.”

In his evidence to MPs, Cummings accused Hancock of being obsessed with meeting a “stupid” target he set himself to offer 100,000 Covid tests a day and of diverting officials’ attention away from the task Cummings had set them to build a test-and-trace scheme from scratch capable of processing 1m tests a day.

Recalling a major battle in Whitehall, Cummings said he had to call around and tell people “do not do what Hancock says, build the thing properly for the medium-term”. Meanwhile Hancock was telling them to “down tools on this” and “hold tests back so that I can hit my target” in order for him to crow about his success in TV interviews.

“He should have been fired for that thing alone,” said Cummings. “It was criminal, disgraceful behaviour that caused serious harm.”

He said he pushed for Hancock to be replaced “almost every week, sometimes almost every day” but that Johnson was advised to keep him in post “because he’s the person you fire when the [public] inquiry comes along”.

Cummings also said he warned the prime minister in February and March that if Hancock was not fired, “we are going to kill people and it’s going to be a catastrophe”.

He said the prime minister came close to sacking Hancock in April “but just fundamentally wouldn’t do it”. Cummings added that he pushed again for the health secretary to be replaced over the summer “otherwise we’re going to have another catastrophe on our hands” in the autumn – but was once again ignored.

Turning to the times Cummings said Hancock lied, he recalled that the health secretary blamed Simon Stevens, the chief executive of NHS England, and Rishi Sunak, the chancellor, for a shortage of personal protective equipment, claiming they had “blocked approvals”. Cummings said he asked Sedwill to investigate to find out if that was correct, and that Sedwill later told him it was “completely untrue”, meaning he had “lost confidence in the secretary of state’s honesty in these meetings”.

Asked if he made a note of Sedwill’s findings, Cummings said yes and promised to supply proof to the two committees quizzing him.

He said there were numerous other examples, also citing Hancock claiming over the summer that “everyone who needed treatment got the treatment they required”.

Cummings claimed: “He knew that was a lie because he’d been briefed by the chief scientific adviser [Sir Patrick Vallance] and the chief medical officer [Prof Chis Whitty] himself about the first peak, and we were told explicitly people did not get the treatment they deserved. Many people were left to die in horrific circumstances.”

Cummings, who left No 10 in November and made headlines in spring 2020 for trips to Durham and Barnard Castle in lockdown, added he was sorry for not “pulling the emergency string” and challenging government figures and scientists earlier. He said he should have urged them to issue the “stay at home” order that came on 23 March “weeks earlier”.

Challenged later by the Labour leader, Keir Starmer, over the “incredibly serious” claim that Sedwill said he had lost confidence in Hancock’s honesty, Johnson said it was untrue and he had not seen any evidence of that.

Johnson’s spokesperson later said he had “full confidence” in Hancock, but twice refused to deny the prime minister had considered sacking the health secretary. He called Hancock’s 100,000 tests a day target “ambitious” and insisted it “saved lives”.