The government's chief medical officer Chris Whitty has said that coronavirus testing was expanded too slowly at the start of the outbreak, leaving the UK "trying to see our way through the fog" as Covid infections increased.
Professor Whitty said that the failure to get testing going at scale early was the largest of "a long list" of regrets he has about the UK response
In contrst, prime minister Boris Johnson refused to say whether he had any regrets over his government's handling of the pandemic.
Speaking alongside Prof Whitty at the daily Downing Street coronavirus briefing, Mr Johnson said it was "simply too early to judge ourselves".
But the chief medical officer said that the PM's scientific advisers would not wait until the end of the outbreak to work out what had been done badly or well, but were "always looking back" in order to learn lessons as events unfolded.
Asked what his greatest regret was in the handling of the pandemic, Prof Whitty said: "I think there's a long list actually of things which we need to look at very seriously.
"If I was to choose one, it would probably be looking at how we could speed up testing very early on in the epidemic.
"Many of the problems that we had came because we were unable to actually work out exactly where we were, and we were trying to sort of see our way through the fog with more difficulty.
"There are many good reasons why it was tricky, but I think if I was to play things again - and this is largely based on what some other countries were able to do, particularly Germany - I think that's the one thing that we would probably have put more emphasis on an earlier stage.
"There are many others, but I want to highlight that one."
Speaking a day after the government was forced to ditch plans to get all primary pupils back into schools before the summer and amid ongoing questions about its test and trace programme, Mr Johnson pushed aside questions about whether he wished he had done anything differently.
"A lot of the questions this evening have been about what did we get wrong in the past," he told the briefing.
"All I can say is that it is simply too early to to judge ourselves we simply don't know the answer to a lot of these questions.
"But I can tell you we know a lot more now than we did in January or February, or even March, and one thing we really do know about tackling coronavirus is you have to proceed with caution and that's what we're doing."
He added: "Frankly, I think that a lot of these questions are still premature and there are lots of things, lots of data, that we still don't know. And this epidemic still has a long way to go alas, not just in this country but around the world."
But Prof Whitty rejected the idea that it was impossible to assess the success or failure of particular interventions while the outbreak was still ongoing.
It will be "absolutely necessary" to hold a post-emergency review once the UK has got over the epidemic he said.
But he added: "It's not a matter of 'do we ever look back?' We always look back - that is the whole point of having the scientific and medical method. You look back and you say 'What could we have done better and what can we do for the future?'
"So we absolutely should. It's always a question of timing and it's always a question of what can you do now, even in the middle of something.
"And be very clear - we are not at the end of this epidemic not by a long shot. We're in the middle of it. Do we actually wait until all the data comes in, and then look back and say 'These are the right things to do'?"
Chief scientific adviser Sir Patrick Vallance said: "Will we have got everything right? No, for sure we won't. There are some things that we will have got wrong and we need to make sure that we understand what they are, learn from them and get them right next time.
"And that's exactly what we need to do. And we shouldn't be just guessing as to what those things are now."
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