Coronavirus must be taken seriously again or the UK will face a "bumpy ride" in the coming months as infections spike, a medical chief has warned.
Professor Jonathan Van-Tam said the British public had "relaxed too much" over the summer and described the rising number of cases as a "great concern".
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.
There were a further 2,948 lab-confirmed cases of coronavirus in the UK as of 9am on Monday, 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 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".
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.
"Don't kill your gran by catching coronavirus and then passing it on," he told told BBC Radio 1's Newsbeat.
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 on Tuesday.
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
- The seven-day rate of new UK cases has risen to 21.3 per 100,000 people, just above the threshold of 20 cases per 100,000 at which the Government considers imposing quarantine conditions on people travelling to the UK, if recorded in other countries
- Train services across England and Wales were increased to about 90% of pre-pandemic levels on Monday, with operators telling commuters they can "travel with confidence"
- A number of schools have reported confirmed cases of coronavirus, including three around Middlesbrough and one in Suffolk
