The UK faces a "bumpy ride over the next few months" and a second wave of coronavirus "is coming", leading health experts have warned.
England's deputy chief medical officer Professor Jonathan Van-Tam said there has been a rise in coronavirus cases because people have "relaxed too much".
He added the rise is "much more marked" in the 17 to 21 age group, but noted there is a "more general and creeping geographic trend" across the UK.
Dr David Nabarro, the World Health Organisation's special envoy for the global COVID-19 response, said "the virus is going to come back" as "life gets going again".
Asked by Sky News if the country can expect a second wave, he replied: "It's coming.
"I don't like it calling it a second wave, I just say there are going to be more spikes and indeed some surges of cases because the virus hasn't changed.
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"It's the same virus that came and caused so much trouble earlier this year.
"It's just been lurking, we've been very good at holding it back through restricting movement and lockdowns."
He continued: "Now as life gets going again, younger people are going to university, also there's some movement around with holidays and of course work – then I'm afraid it does mean the virus is going to come back."
The UK's weekly rate of new COVID-19 cases has risen above 20 per 100,000 people – the rate at which it considers imposing quarantine measures on people arriving from abroad.
In the seven days to 7 September, there were 21.3 cases per 100,000, and a total of 14,227.
There were 13.9 per 100,000 in the seven days to 31 August, and a total of 9,259.
There were a further 2,948 lab-confirmed cases of COVID-19 in the UK as of 9am on Monday, following the 2,988 reported on Sunday, which was the largest daily figure since May.
Professor Van-Tam said the latest coronavirus figures were of "great concern", adding: "This is a virus we're going to have to live with – and if we're not careful, if we don't take this incredibly seriously from this point on, we're going to have a bumpy ride over the next few months."
Asked what is behind the rise in cases, he replied: "People have relaxed too much."
He added: "Now is the time for us to re-engage and realise that this is a continuing threat to us."
The deputy chief medical officer for England issued the warning as Caerphilly in south Wales prepared to be placed under local lockdown and stricter measures were extended in Scotland.
The health experts warnings come as:
- The total number of confirmed cases in the UK passed 350,100
- The seven-day rate of new UK cases has risen to 21.3 per 100,000 people
- Train services across England and Wales were increased to about 90% of pre-pandemic levels on Monday
- A number of schools have reported confirmed cases of coronavirus, including three around Middlesbrough and one in Suffolk
- The government's testing website said at 11pm on Monday that there were no more home testing kits available that day
Professor Van-Tam has urged politicians and public health officials to think how to manage the crisis, not in the short term, but through "the next six months and how we get through this until the spring".
The professor added that it was "clear" that the level of compliance with restrictions "is very variable indeed".
Health Secretary Matt Hancock welcomed the "important advice", having earlier described the recent increase in cases as "concerning", as he tried to remind young people of the dangers of the situation.
He told BBC Radio 1's Newsbeat: "Don't kill your gran by catching coronavirus and then passing it on."
It comes as Housing Secretary Robert Jenrick told Sky News there seems to be a rise in the number of cases among young people.
He continued: "Those people… who are out enjoying themselves, who might be going to university this month – they do need to exercise a degree of caution.
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