The number of deaths from coronavirus could have been halved if lockdown was introduced a week earlier, an expert who advised the Government on restrictions has said.

Neil Ferguson, professor of mathematical biology at Imperial College London, told the Science and Technology Committee thousands of deaths could have been prevented with earlier action.

However, he explained that based on what was known about transmission and fatalities at the time, the actions taken were warranted.

Prof Ferguson said: “The epidemic was doubling every three to four days before lockdown interventions were introduced.

“So, had we introduced lockdown measures a week earlier, we would have reduced the final death toll by at least a half.

“So whilst I think the measures, given what we knew about this virus then in terms of its transmission and fatality, were warranted, certainly had we introduced them earlier we would have seen many fewer deaths.”

The UK was put into lockdown on March 23 in an unprecedented step to attempt to limit the spread of coronavirus.

Early in the outbreak, experts had estimated that the number of coronavirus deaths in the UK would be unlikely to exceed 20,000.

When asked what had gone wrong, Prof Ferguson said: “I think two things – one is a paper actually out in Nature, which highlights that around about that time, just before lockdown happened, the first two weeks of March, we probably had 1,500 to 2,000 infections imported from Italy and Spain, which we just hadn’t seen in the surveillance data, until that point.

“So there is much heavier seeding than we’d expected.”

He added: “The key things to determine number of deaths is at what point in your local epidemic you trigger interventions – how far in are you when you shut down transmission.

“And we frankly had underestimated how far into the epidemic this country was, that’s half the reason.

“The second part, which I think would have been more avoidable, is about half of those deaths occurred in care homes.”

Prof Ferguson continued: “And we did all this working under the assumption which was Government policy at the time that care homes would be shielded from infection.”

He further told the committee: “We also made a rather optimistic assumption that somehow – which was policy – that the elderly would be shielded, the most vulnerable would be shielded as the top priority. And that simply failed to happen.”

Other experts said Prof Ferguson’s comments were “a simple statement of the facts as we now understand them”.

James Naismith, professor of structural biology at the University of Oxford, added: “During the exponential phase of the virus, even a few days can make a big difference. The UK’s significantly higher death toll than Germany is most likely down to difference in the timing of the lockdown.

“The brutal truth is we lacked the testing capability as the virus took hold here. The increased testing capability now on stream and the developing track and trace system should combine to ensure we much better understand any second wave and therefore act much more quickly.”

Prof Ferguson, whose research helped usher in the lockdown, resigned from his role as a key Government adviser after admitting that he had undermined social distancing rules by reportedly meeting a woman at his home.

However, he told the committee he still sits on SPI-M, which advises the Scientific Advisory Group for Emergencies (Sage).

Prof Ferguson told the committee that despite border screening at the early stage of the pandemic, probably 90% of cases imported into the country were missed because checks were not being conducted on people arriving from Europe.

Greg Clark, chairman of the committee, asked Prof Ferguson if that was because Sage had advised that it was not necessary to test those people.

Prof Ferguson said: “This is really decisions by the Foreign Office and by the Department of Health and Social Care, not by Sage.

“Sage recommended that where a country had been identified as having an active transmission, we should check travellers from those countries.”

He added that the difficulty was that Spain and Italy – the source of many infections into the UK – had large epidemics before they realised.

“We were just not aware of the scale of transmission in Europe as a whole,” Prof Ferguson said.

The committee also heard that had the UK had the testing capacity at that stage, then at least screening everybody with symptoms arriving in the country would have given a better impression of where infection was coming from.

The Imperial College Covid-19 response team called for a full-scale lockdown in a paper on March 16 – seven days before it was implemented in the UK.

When asked if he was disappointed in the delay, Prof Ferguson said: “I said earlier, but in retrospect I would have much preferred it to have been taken a week earlier given that many lives would have been saved.”

I think a variety of different people are going to come with different answers on the timing question

Prof Ferguson’s comments were put to Prime Minister Boris Johnson, chief scientific adviser Sir Patrick Vallance, and chief medical officer Professor Chris Whitty at the daily Downing Street press conference.

Sir Patrick said it would be important to look back and see what measures had been most effective, and would be useful in determining what measures to implement in the future, if necessary.

He added: “So I think those are important scientific questions to address, and they haven’t yet been fully addressed.”

Prof Whitty said it was important to learn lessons from what had happened in the outbreak.

“I think a variety of different people are going to come with different answers on the timing question,” he said.

“Part of the problem that we had at that stage is that we had very limited information about this virus.”