Britain’s whole system of critical national infrastructure remains “wildly under-resilient”, the Covid-19 inquiry has been told by Oliver Letwin, who was responsible for emergency planning in the coalition government.

It was also an “error” that no government had appointed a senior minister with sole responsibility over planning for pandemics and other areas of resilience, the former minister said. He described the churn of ministers and officials tasked with preparing for emergencies as a “disaster for the country”.

Letwin said resilience was a “relatively small part” of his brief while Cabinet Office minister in charge of it between 2011 and 2016, having taken on the resilience portfolio in late 2011.

“There really ought to be a minister solely devoted to resilience at a senior level,” said Letwin, who was giving evidence ahead of George Osborne, the chancellor in the coalition government, who has been the target of criticism that his austerity policies left the UK underprepared for the Covid-19 pandemic.

Letwin was asked by counsel for the inquiry about the “longstanding bias” towards planning for a flu pandemic and was shown a briefing note that raised the issue of whether stockpiles of antibiotics were sufficient for non-influenza pandemics.

He replied that he had been aware of it and that there had been a consensus in the Department of Health and the Health Protection Agency that this was “under control”, something that was “ludicrous in retrospect”.

Osborne rejected suggestions that his austerity programme while chancellor depleted health and social care capacity and insisted the cuts had made Britain better prepared to tackle the coronavirus pandemic.

“If we had not had a clear plan to put the public finances on a sustainable path then Britain might have experienced a fiscal crisis, we would not have had the fiscal space to deal with the coronavirus pandemic when it hit,” said Osborne.

In written evidence, he argues that his action “had a material and positive effect on the UK’s ability to respond to the Covid-19 pandemic”.

Under questioning, the former Conservative MP said he did not think it was “particularly fair to apportion blame” when it came to the question of why the Treasury had not planned for a lockdown, adding that scientific experts had focused on the need to prepare for a flu rather than a coronavirus pandemic.

“No one said to us there could be a health pandemic that is not influenza, for which the likely response is you are going to shut down the economy for months on end,” he said.

Later in his evidence, Osborne said: “I wonder, if we had done a tabletop exercise in 2012, that we would have come to the conclusion that you could lock down the entire population, that it could be a feasible option.”

Kate Blackwell KC, a barrister for the inquiry, replied: “Well, we will never know because it was never done, was it?”

Asked whether he saw no connection between austerity and Covid disproportionately affecting the most disadvantaged people, Osborne said: “That’s absolutely my contention.”

“It is true that pandemics will affect poorer people more severely and that is one of the great tragedies, which is why we tried to alleviate poverty and direct services towards them.”

“I think everything we did to try and ring fence the NHS budget, to provide stable finances so that they were not further affected by a fiscal crisis, things like universal credit which were introduced, all of these things were done to try and protect the poorest part of the population.”

Paul Nowak, the general secretary of the Trades Union Congress, which has argued that the “political choice” of austerity under the coalition left the UK “hugely exposed to the pandemic”, later accused Osborne of “trying to rewrite history and gaslight the British public”.

On Monday, David Cameron admitted failures in the coalition government’s preparations for a pandemic but also defended the austerity drive that he and Osborne had imposed, saying: “Your health system is only as strong as your economy.”

Prof Philip Banfield, the chair of the British Medical Association, has said there was “no doubt that both staff and patients were put in harm’s way” because of underfunding in the decade running up to the start of the pandemic.