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Two members of the UK government's Scientific Advisory Group for Emergencies (Sage) have given stark warnings over the increase in coronavirus cases across the country.
Prof John Edmunds said cases were now "increasing exponentially".
While England's deputy chief medical officer, Prof Jonathan Van Tam, said people had "relaxed too much" and must start taking the virus seriously again.
Some 2,948 UK cases were recorded on Monday, according to government data .
It follows 2,988 new cases being announced on Sunday, which was the highest figure since 22 May.
Prof Edmunds, from the London School of Hygiene and Tropical Medicine, told ITV's Robert Peston that the R number - which rates Covid-19's ability to spread - had risen "above one", so the UK was in a "risky period".
"We can see the epidemic is taking off again. So I don't think we've hit that sweet spot where we've been able to control the epidemic and allow the economy to return to some sort of normality," he said.
Prof Edmunds, who has previously said he wished the UK had gone into national lockdown earlier than it did , added that cases were rising from a "relatively low level" but "are increasing now exponentially".
Earlier on Monday, Prof Van Tam described the latest "big change" in coronavirus infections across the UK as a "great concern" .
"People have relaxed too much. Now is the time for us to re-engage, and to realise that this a continuing threat to us," he said.
Prof Van Tam added that hospital admissions and deaths were "at a very low level" in the UK and the rise in cases was most prominent among those aged between 17 and 21 - but the country risks following the trajectory of some EU countries.
"Where case numbers rise initially in the younger parts of the population they do in turn filter through and start to give elevated rates of disease and hospital admissions in the older age groups, and we know that then becomes a serious public health problem," he said.
"The fact that 17 to 21-year-olds are not becoming ill means they are lucky, but they also forget because the disease is not severe for them that they are potent spreaders."
He said the UK would be in for "a bumpy ride over the next few months" if people did not adhere more stringently to social distancing rules.
Wake-up call for the public
Prof Jonathan Van Tam was known for his plain speaking at Downing Street press conferences earlier in the pandemic.
Now England's deputy chief medical officer has once again come out with a blunt warning about the spread of the virus.
Put simply, he believes there's been a significant and concerning change in recent days. No longer is this all about local outbreaks - instead, in his view, there is now a general spread of the disease in many parts of the country.
He suggests social distancing has broken down for some parts of society and points to rising case numbers amongst 17 to 21-year-olds.
That does not mean hospital admissions, currently relatively low, will definitely rise. But Prof Van Tam thinks that is very possible.
He implied the next week would be critical as officials and ministers studied the emerging data.
This is a wake-up call for the public to get real about social distancing, from a medical leader who is clearly worried.
The Sage scientists' comments come as more parts of the UK are to face tougher restrictions following a rise in the number of cases.
Stricter rules on home visits were extended to two more areas in the west of Scotland from midnight.
In Wales, the county borough of Caerphilly is to be placed under a local lockdown from 18:00 BST on Tuesday.
Meanwhile, council leaders in north-east England have said the average number of new cases in the region has doubled to about 80 per day, in just over a week.
A joint statement from seven council leaders said: "We have seen cases where individuals with symptoms have had a test, then gone out and infected others before getting their results - reckless and selfish behaviour."
The statement also warned businesses that they would be shut down if they did not record customers' details to aid the NHS Test and Trace contact tracing programme.
"We all have to do our bit and to play our part if we are to prevent a potential second wave," the leaders said.
- THE R NUMBER: What it means and why it matters
- TESTING: What tests are available?
- LOOK-UP TOOL: How many cases in your area?
- LOCAL LOCKDOWNS: What happens if you have one?
- SOCIAL LIFE: Is it safe to go to the pub?
The warnings come after Transport Secretary Grant Shapps announced a change in England's quarantine policy, adopting an approach which allows islands to have separate rules from a country's mainland.
He said travellers arriving in England from seven Greek islands will have to self-isolate for 14 days from 04:00 BST on Wednesday.
They are Crete, Lesvos, Mykonos, Santorini, Serifos, Tinos, and Zakynthos (also known as Zante).
- THE NAKED SCIENTISTS: Where did COVID-19 come from?
- THE SECRET LIFE OF TEACHERS: What's it like returning to classrooms amid a global pandemic?
from Via PakapNews