The Government’s top scientific adviser has criticised a “lack of appreciation that very quick decisions were needed” during the pandemic.

Prof Dame Angela McLean, the Government’s chief scientific adviser, said this was the “most significant shortcoming” in decision-making during the pandemic.

Dame Angela told the UK Covid-19 Inquiry there was a “lack of appreciation that very quick decisions were needed” around exponential growth of cases and “lagged control” – the time-lag between something happening and its actual effect.

She said the aim was to “stop too many people being admitted to hospital” but people were generally admitted 10 to 11 days after infection, showing there was a time-lag.

“So if you wait until the thing you’re worried about is really, really bad and growth is exponential and fast, you can very easily end up with things twice as bad at the hospital door, even if you put in a brilliant intervention,” she said.

“So this idea that things were – whatever your control measure is – doesn’t fix your problem until 10 days later, is a really important part of why this was such a difficult problem.

“And really, as I point out… (it) needed to be understood, because it made watch and wait tactics very damaging.”

She agreed that the failure to take a decision is “a positive decision in itself when it comes to exponential growth”.

Joanne Cecil, counsel to the inquiry, asked her: “And as we will see as we move chronologically through that period, that informs what you consider to be one of the most significant shortcomings in relation to decision-making… to some extent in the early part of the pandemic, but certainly in the autumn period of 2020?”

Dame Angela replied: “I agree. I think we made the same mistake three times.”

She said understanding the concepts did not require a scientific mindset.

Meanwhile, Dame Angela said that there was enough data for the Government to initiate the first Covid lockdown a week before it was actually introduced.

She told the inquiry that in mid-March 2020 ministers had enough information to show them that voluntary measures had only reduced social contact by around 40%, when experts had said that contacts needed to reduce by around 75% to reduce the spread of the virus.

“We knew that we needed about 75% reduction in contact,” she said.

“So I think this is something I think is quite important to remember that we did actually have data that very day that said the voluntary reductions were not enough.”

Asked whether the lockdown announcement was timely, she added: “You’ve already heard from colleagues that it was too late.

“So if we’re doing a ‘with benefit of hindsight exercise’ here, I would say it should have been two weeks earlier, that that would have made a really huge difference.

“Now we didn’t have the data, two weeks earlier.

“By the 16th we had enough data, in my opinion, we should have gone into lockdown on that Monday the 16th.

“On the 16th – given what we knew about how fast this epidemic was spreading, given what we knew and could surmise about the fact that probably everybody was susceptible to catch it – I think there was enough information on that date to say we need to stop all non-essential contact.”