Professor Steven Riley, 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 Riley, who now works for the UK Health Security Agency, said subsequent data had shown that people began to change 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."

Meanwhile, Professor Neil Ferguson - whose COVID modelling was instrumental to the UK going into lockdown - denied stepping "over the line" and telling ministers they needed to shut down.

Prof Ferguson, who is director of the school of public health at Imperial College London and was a member of the Scientific Advisory Group for Emergencies (SAGE) during the pandemic, was asked whether he had become involved "in the determination of policy".

Prof Ferguson replied: "I know I'm very much associated with a particular policy... but the reality was a lot more complex.

"I don't think I stepped over that line to say: 'We need to do this now'.

"What I tried to do was, at times - which was stepping outside the scientific advisory role - to try and focus people's minds on what was going to happen and the consequences of current trends."

Imperial College produced a wide range of models during the COVID pandemic.

One suggested that in the worst-case scenario, 65,000 Britons could die, while another suggested 500,000 would die if no action was taken to tackle the spread of the virus.

The World Health Organisation declared a global pandemic on 11 March 2020.

There have been more than 230,000 COVID-related deaths in the UK, according to the latest official data.

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

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".

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It comes as evidence submitted to the inquiry by a leading thinktank 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".