A scientist who advises the government on coronavirus says he wishes the UK had gone into lockdown sooner as the delay "cost a lot of lives".
But Prof John Edmunds said data available in March was "really quite poor", making it "very hard" to do so.
Health Secretary Matt Hancock says the government "took the right decisions at the right time".
It comes as the UK recorded the deaths of another 77 people who tested positive for coronavirus.
It marks the lowest daily increase in the number of deaths since 24 March.
Scotland recorded no new coronavirus deaths for the first time since lockdown began, while Northern Ireland reported no new deaths for a second time.
The number of coronavirus deaths recorded over weekends has tended to be lower because of reporting delays.
Meanwhile, Mr Hancock said the government had reached its target this weekend of delivering tests to all staff and residents of care homes.
The government said it had offered coronavirus test kits to every care home in England and had delivered tests to 9,000 eligible care homes.
But Labour's shadow minister for social care, Liz Kendall, said the original pledge had been for tests to have been carried out, not just delivered to care homes, and accused the government of being "too slow to act".
The pledge was made on 15 May, when Mr Hancock said all residents and members of staff in care homes in England would have been tested for coronavirus by early June.
'Broad range of opinion'
Asked on the BBC's Andrew Marr Show whether he had any regrets about his advice during the epidemic, Prof Edmunds said: "Yes, we should have gone into lockdown earlier.
"I think it would have been hard to do it, I think the data that we were dealing with in the early part of March and our situational awareness was really quite poor.
"So I think it would've been very hard to pull the trigger at that point, but I wish we had.
"I wish we had gone into lockdown earlier. I think that has cost a lot of lives unfortunately."
Asked about the epidemiologist's comments, the health secretary defended the government's announcement of the lockdown on 23 March, saying: "I think we took the right decisions at the right time."
Mr Hancock said there was a "broad range… of scientific opinion" on the government's Scientific Advisory Group for Emergencies (Sage) and Prof Edmunds was one among more than 100 members.
"We were guided by the science - which means guided by the balance of that opinion - as expressed to ministers through the chief medical officer and the chief scientific adviser," he added.
"That's the right way for it to have been done."
Mistakes were undoubtedly made
Looking back now, it is clear the virus was much more widespread than was realised in February and March.
It is estimated that by the time lockdown was announced on 23 March, there were 100,000 new cases a day.
At the time, testing and surveillance was picking up only a small fraction of them. When the scale of the outbreak was realised, scientists advising the government pushed for lockdown - and ministers subsequently agreed.
It is easy to criticise both the failures of science and the decisions of ministers in hindsight.
Other countries had already moved to lockdown ahead of the UK, but still we held out for a few weeks.
The key question is, should we have known more at the time and should we have been better prepared?
This is all likely to be pored over in a public inquiry at some point and that will no doubt show mistakes were made - the government's chief scientific adviser, Sir Patrick Vallance, has admitted as much himself.
Mr Hancock told the BBC's Andrew Marr the government would take a "very cautious and safety first" approach to further relaxation of the rules.
The lockdown has already been eased slightly, with changes made across the UK to meeting others outdoors and English schools reopening for some pupils.
Further easing is due from 15 June in England, with non-essential retailers allowed to re-open and places of worship allowed to open for private prayer.
Labour's shadow foreign secretary, Lisa Nandy, said her party supported easing the lockdown but called for improvements in the government's test and trace system.
R number 'creeping up'
Downing Street said any alterations to lockdown measures in the coming weeks would depend on the government's five tests continuing to be met, in order to prevent a second spike in the virus and stop the NHS being overwhelmed.
These include ensuring the R number - the number of people an infected person passes the virus onto on average - stays below one.
Speaking to Andrew Marr, Prof Edmunds warned R was "creeping up" in some places - with some reports suggesting it had gone above one in north-west England.
But Mr Hancock said the estimated R was "below one in each region", and the government would "take local action in the first instance to crack down on any local outbreak" - including reintroducing lockdown measures.
The Labour mayors for Greater Manchester and Liverpool, Andy Burnham and Steve Rotherham, said talk of local lockdowns were "not helpful" and said it was "unacceptable" to not include them in the planning process.
They said if the government was "determined to proceed" with the measure, "significant support needs to be put in place" for English regions, including a local furlough scheme and funding for councils.
Meanwhile, the prime minister is set to announce more easing of lockdown measures for 15 June, with Boris Johnson expected to tell cabinet about the additional changes on Tuesday.
It has also been revealed the PM plans to give a speech in the summer, setting out his vision for how the UK can recover from the coronavirus crisis.