Another pandemic as big as the Covid crisis that killed 7 million people worldwide is “a certainty”, Prof Sir Chris Whitty has warned, as he said that the UK’s lack of intensive care capacity for the sickest patients was a “political choice”.

The NHS faced an “absolutely catastrophic situation” when the virus first hit in 2020 but it could have been “substantially worse” if the UK had not gone into lockdown, England’s chief medical officer said.

“We have to assume a future pandemic on this scale will occur,” he told the public inquiry into Covid-19 on Thursday. “That’s a certainty.”

It would also be “foolish” not to assume that asymptomatic transmission of a deadly virus would happen again, he added.

The warning from Whitty came after a doctor repeatedly broke down in tears at the inquiry while describing how the Covid crisis for NHS staff had been like having to respond to a “terrorist attack every day”, with infected patients “raining from the sky”.

Prof Kevin Fong – a former clinical adviser in emergency preparedness, resilience and response at NHS England who was on shift during the 7/7 London bombings – said the scale of deaths in hospitals at the height of the pandemic had been “truly astounding”.

Some intensive care units in England had been so overwhelmed that staff had had to put dead bodies in clear plastic refuse sacks after running out of body bags, and then immediately put another Covid patient in that person’s bed, Fong said.

Giving evidence after Fong, Whitty said expanding the capacity of the NHS could help it cope better with a surge in sick patients in future. “Taking ICU [intensive care units] in particular, the UK has a very low ICU capacity compared to most of our peer nations in high-income countries,” he said. “Now, that’s a choice, that’s a political choice.

“It’s a system configuration choice, but it is a choice. Therefore you have less reserve when a major emergency happens, even if it’s short of something of the scale of Covid.”

Whitty said resolving the NHS workforce crisis was also crucial. Healthcare systems could not be “scaled up” in a future pandemic without “trained people”, he added.

“You can buy beds, you can buy space, you can even put in oxygen and things … But fundamentally, the limit to that system, as to any system, is trained people, and there is no way you can train someone in six weeks to have the experience of an experienced ICU nurse or an experienced ICU doctor. It is simply not possible.

“So if you don’t have it going into the emergency – if it’s an emergency of this speed of onset – you should not have any illusions you’re going to have it as you hit the peak.”

Among his other recommendations, Whitty said being able to conduct lightning-fast scientific research and reducing health inequalities deserved the most focus. “If we’re not serious about trying to tackle health inequalities between pandemics, there is no way you’re going to be able to do it when the pandemics occur,” he said.

“The biggest one that I think deserves slightly more emphasis is having the mechanism to be able to do research very, very fast.”

Referring to the development of vaccines and treatments, he added: “I think people always, at the beginning of pandemics, underestimate that it’s actually the science that’s going to get them out of the hole, not all the other things. The other things are holding the line until the science does the work.”

Whitty said that, as well as direct harms from Covid such as deaths and long Covid, there were indirect harms “that come from the system being overwhelmed or at least unable to cope … all diseases, not just Covid, having higher mortality rates than they would have had”.

He admitted that messaging around what masks NHS staff should wear to protect themselves against Covid had been “quite confused” early in the pandemic. He also said officials “didn’t get it across well enough” that the public should continue to go to hospital for serious illnesses other than Covid. There was never going to be “perfect balance” when it came to stay-at-home messaging, he said.

“We tried … we tweeted, we talked over the podium and so on. Whether we pushed too strongly on the messages the risks of Covid, I think that’s a much harder one, actually.”

Whitty said he still worried today about whether the government got “the level of concern right” about the dangers of Covid in 2020. “Were we either overpitching it so that people were incredibly afraid of something where in fact, their actuarial risk was low, or were we not pitching it enough and therefore people didn’t realise the risk they were walking into?

“I think that balance is really hard, and arguably some people would say we – if anything – we overdid it, rather than under, in the beginning.”

In a statement, Covid-19 Bereaved Families for Justice UK, which represents thousands of people, said the scenes described in intensive care units “were not inevitable, can never be considered acceptable, and must never be repeated”.