And he said that by early March, it was clear that the development of the outbreak was pointing towards that worst-case scenario coming true if no action was taken.
The government’s decision to impose the first lockdown in March last year is widely understood to have been driven by an Imperial College London study that month which said as many as 500,000 could die if the virus was allowed to spread unhindered while the UK developed “herd immunity”.
But giving evidence to an inquiry by the House of Commons health and science committees, Mr Hancock revealed that the higher figure - calculated by the Department of Health on the basis of infection and death rates in the Spanish flu outbreak of 1918, when 228,000 died in the UK - was already known to ministers on the government’s Cobra emergencies committee.
“I asked for a reasonable worst-case scenario planning assumption, and I was given the planning assumption based on Spanish flu,” Mr Hancock told MPs.
“It was signed off at Cobra on 31 January, and that was a planning assumption for 820,000 deaths, and I was determined that that would not happen on my watch.”
Mr Hancock added: “In the middle of February the scientific advice confirmed that the reasonable worst-case scenario should be taken as read that this was equivalent to Spanish flu.
“At the time at the end of January when that was first presented at Cobra, I - like everybody else - thought of Spanish flu as something you’ve read about in the history books. But as health secretary, you’re always worried about new pathogens.
“Knowing that that was the reasonable worst case scenario, we planned for it.”
Mr Hancock told MPs: “The week beginning 9 March, what happened is that the data started to follow the reasonable worst-case scenario. And by the end of that week, the updated modelling showed essentially that we were on the track of something close to that reasonable worst-case scenario.”