Professor Steven Reilly, who worked for Imperial College London at the time of the pandemic, told the UK COVID-19 Inquiry that the government should have called the lockdown on 9 March 2020 instead of 23 March.

Prof Reilly, who now works for the UK Health Security Agency, said subsequent data had shown that people began to alter their behaviour on or around 16 March, a few days before the public was ordered to stay at home.

"My view is that the first national period of stringent social distancing [lockdown] should have been introduced on or around 9 March 2020," he wrote in his witness statement to the inquiry.

Asked to elaborate, he told the inquiry: "Once we had lab confirmed deaths in ICUs [intensive care units] with no travel history, no obvious connections to any out of country social networks, even a handful of those would indicate that we would be rapidly progressing in our epidemic.

"We've got a lot of data about how social mixing changed over this period and actually on or around 16 March seems to be when everybody did start to change their behaviour."

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He added: "So I think the best way to talk about this is to say: had we achieved that rapid reduction in mixing earlier than the 16th then the peak height would have been lower, and the area under the curve for the first wave would have been less, and potentially quite a bit less."

The COVID-19 Inquiry began this summer and has so far heard evidence from significant political figures, including former health secretary Matt Hancock and former prime minister David Cameron.

The second part of the public inquiry - which focuses on "core decision making and political governance" - started at the beginning of this month and will see Boris Johnson and closest aide Dominic Cummings give evidence.

The inquiry heard how Mr Johnson described long COVID as "b*******" and that his wife, Carrie, had been described as "the real person in charge" by the head of the UK's civil service.

A document from October 2020 described the symptoms of the condition, beside which the-then prime minister wrote "b*******" and "this is Gulf War Syndrome".

Mr Johnson repeated similar remarks in a WhatsApp message four months later, the inquiry heard.

WhatsApp exchanges disclosed at the inquiry last week showed the head of the civil service, Simon Case, had described Mr Johnson's wife Carrie as "the real person in charge".

Mr Case, who was appointed in September 2020 having previously served as permanent secretary in Number 10, also said the government was looking like a "terrible, tragic joke" and that he "cannot cope with this" in apparent frustration at how the pandemic was being handled.

The messages briefly appeared on a screen during a hearing of the inquiry, as a letter to the inquiry from Mr Cummings was shown and discussed.

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It comes as evidence submitted to the inquiry by a leading think tank suggested that the NHS and social care in the UK is still "highly vulnerable to future shocks" to the system.

The Health Foundation said a "lack of health service capacity constrained the response to COVID-19" as it warned that "without sustained investment in increasing resilience, response to future health threats are likely to be similarly hampered".

It also found the UK entered the pandemic with fewer doctors, nurses, hospital beds and equipment compared to similar countries, and that funding growth for the NHS had been "severely constrained" before COVID hit.

"The pre-existing constraints in our health and care system risk prolonging the recovery of services after the pandemic and, without sustained investment, leaves the UK highly vulnerable to future shocks," the authors wrote.