Video report by ITV News Science Editor Tom Clarke
The chancellor has assured the public it is "absolutely safe" to enjoy new freedoms provided by the government's easing of lockdown restrictions in England.
As some restrictions were relaxed on Monday - to allow outdoor meetings of up to six people, as well as the reopening of outdoor retail and primary schools - a number of scientists and public health leaders said lockdown is being eased too fast.
But Rishi Sunak, visiting an outdoor market in London, said "it's safe and people should have the confidence to go out there and get their life a little bit back to normal".
He said "great progress" had been made in previous weeks, which meant a "really important first step" could be made to allow outdoor retail to reopen.
"I'm here grabbing my lunch, people are here doing the same, traders are all back - I hope in the coming days and weeks many more people will do the same and slowly we're going to get our lives back to normal," he added.
However number of top scientists, who sit on Sage - the committee advising the government about the coronavirus pandemic - and the Association of Directors of Public Health (ADPH) are warning the move to relax lockdown has been made too soon.
The ADPH said it was "increasingly concerned" about the new rules coming into force, saying they are "not supported by the science" as the government has consistently claimed.
It said pictures of crowded beaches and beauty spots over the weekend showed "the public is not keeping to social distancing as it was" and the NHS test and trace programme "is currently far from being the robust operation that is now urgently required as a safeguard to easing restrictions".
ITV News Science Editor Tom Clarke has said he 'worries that this might be the departure between the scientific advise and the political realities of easing lockdown'
But the government has insisted lockdown easing "is not a dash" and any changes are being made in a "very cautious" way.
Following a host of easing measures, groups of up to six people are now allowed to meet outdoors and in private gardens in England, those in the clinically extremely vulnerable can now go outdoors, and competitive sport is set to resume.
Dr Jeanelle de Gruchy, president of the ADPH, told Radio 4's Today programme there was a need to balance easing of restrictions with the risk of causing a resurgence in infections.
"A lot of people including local directors of public health across the country are increasingly concerned that the government is misjudging this balancing act and lifting too many of the restrictions too quickly," she said.
"The five tests haven't yet been met.
"In terms of the R (a measure of infections produced per person) it's 0.7 to 0.9, in the latest government assessment it is below one, but it's a very limited room for manoeuvre and we know how quickly this virus can spread."
But Business Secretary Alok Sharma insisted that the changes "are cautious".
Mr Sharma told ITV News: "The measures we are taking are indeed phased, they are cautious, and they are proportionate".
"We are doing this in a phased manner, and actually that is what the scientific advisory group of emergencies [...] has also said that we should do".
He stressed "only certain" schools and classes within them are returning from Monday in England.
It comes as one survey suggested nearly half (46%) of families plan to keep pupils at home as classes in England open to children in Reception, Year 1 and Year 6.
A number of scientists have also voiced their concerns at the easing of lockdown, Professor Peter Openshaw, a member of NERVTAG (the government's new and emerging respiratory virus threats advisory group) told ITV News some are "feeling nervous" about the changes.
"In some parts - particularly in the North of the country - there are still a lot of cases around in the community.
"If people start ignoring government advice [...] and thinking it's perfectly aright to go about their normal business [...] there could be a big resurgence.
"We could experience a second major lockdown".
Professor Peter Openshaw says there is still 'a significant risk':
It's a concern echoed by Dr Gruchy of the ADPH, who said the country is not ready for a second wave of Covid-19.
"We're concerned about meeting all the other operational challenges ready to meet a potential rise in infections," she said
Adding that the national testing programme and the test and trace scheme must be "robust and ready" and the personal protective equipment (PPE) supply must be adequate.
"We're not feeling just yet that we're confident enough to meet any potential challenge if the government goes too quickly on easing lockdown measures," she added.
ITV News Correspondent Greg Easteal reports from Thirsk where the market reopened since measures were lifted:
In a blog on Sunday, the ADPH said: "Over the weekend we have seen signs that the public is no longer keeping as strictly to social distancing as it was.
"Along with this, we are concerned that the resolve on personal hygiene measures, and the need to immediately self-isolate, if symptomatic, is waning.
"A relentless effort to regain and rebuild public confidence and trust following recent events is essential."
It's unclear if the group is referring to the row that erupted following revelations about the travel of Boris Johnson's chief aide Dominic Cummings who drove 260 miles from London to Durham during lockdown.
The blog said the government's second test from its list of five - a sustained and consistent fall in the daily death rate - was happening, but "the downward trend is slow particularly in care settings".
The impact of easing restrictions on the R value was difficult to judge given they were being eased in a rapid way, it added.
"As we saw in March, R can go above one in a very short space of time - and once it does it can take many months to bring it back down."
It warned: "The room for manoeuvre is tight."
The ADPH also said a second peak cannot be ruled out, and asked: "Do we really want the same number of deaths again?
"The scale to date represents an unimaginable tragedy and we must do everything possible to limit further loss of life."
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