British Prime Minister Boris Johnson has told his cabinet team of top ministers that they must work to stop the spread of the coronavirus from the young to older people to prevent hospitalisations, his spokesman said.
The cabinet were updated on the coronavirus response by chief medical officer Chris Whitty and chief scientific adviser Patrick Vallance after an increase in cases.
"The prime minister cautioned that in other countries which had seen an increase in infections this was followed a number of weeks later by a rise in hospitalisations," his spokesman said.
"The PM said that what had taken place elsewhere was that young people had gone on to infect older generations, who had become seriously ill, and it was vital to ensure that did not happen here," he said, adding that Mr Johnson said the government must remain "extremely vigilant".
Health Secretary Matt Hancock told the House of Commons "this virus is still very much with us and remains a threat".
He said that "just because we've come through one peak, it does not mean we cannot see another one coming" following a rise in coronavirus cases in the UK.
In a statement, he said: "Today's ONS figures show that the weekly coronavirus deaths have dropped to their lowest number since mid-March and the latest daily number of recorded deaths is three.
"However, we have seen a concerning rise in the number of positive cases, particularly among younger people, and these figures serve as a salutary reminder that this virus is still very much with us and remains a threat."
Earlier, Professor Jonathan Van-Tam, deputy chief medical officer for England said coronavirus must be taken very seriously again or the UK will face "a bumpy ride over the next few months".
Mr Van-Tam said that the public had "relaxed too much" over the summer and described the rising number of cases were of "great concern".
He issued the warning as Caerphilly in south Wales prepared to be placed under local lockdown and stricter measures were extended in Scotland.
There were a further 2,948 lab-confirmed cases of coronavirus in the UK as of yesterday morning, following the 2,988 reported on Sunday, which was the largest daily figure since May.
In an interview with journalists, Prof Van-Tam said: "This is a big change. It's now consistent over two days and it's of great concern at this point.
"We've been able to relax a bit over the summer, the disease levels have been really quite low in the UK through the summer but these latest figures really show us that much as people might like to say 'oh well it's gone away' - this hasn't gone away.
"And if we're not careful, if we don't take this incredibly seriously from this point in we're going to have a bumpy ride over the next few months."
He said that the rise is "much more marked" in the 17-21 age group, but noted there is a "more general and creeping geographic trend" across the UK.
"People have relaxed too much," Prof Van-Tam said. "Now is the time for us to re-engage and realise that this is a continuing threat to us."
He 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".
Professor John Edmunds, who is part of the government's Scientific Advisory Group for Emergencies, warned that cases were "increasing exponentially".
He said the UK has entered "a risky period" with the average number of people an infected individual spreads the virus to, known as the reproduction number, potentially above the crucial figure of one.
"I didn't want us to relax measures so much that we couldn't open the schools safely without it tipping the reproduction number significantly above one. And we are already above one and we've opened schools," he told ITV News.
As the UK continued attempts to quell regional outbreaks, people were told they cannot enter or leave Caerphilly without a reasonable excuse when new restrictions are imposed at 6pm today.
Meetings with other people indoors will be banned and everyone over 11 will have to wear masks in shops, the first time the measure has been made mandatory in Wales.
The South Wales borough has seen 133 new Covid-19 cases over the past seven days, equivalent to a rate of 55.4 cases per 100,000 population, giving it one of the highest rates in the UK.
Meanwhile, restrictions on household visits across western parts of Scotland were extended for a further week and expanded to also include East Dunbartonshire and Renfrewshire.
The developments came as the total number of confirmed cases in the UK passed 350,100.