The United Kingdom registered almost 3,000 new coronavirus cases over 24 hours, a level not seen since late May, health ministry figures showed Sunday.
The government said two more people had died after testing positive over the past 28 days, bringing the overall UK toll to 41,551, Europe's highest. In addition, 2,988 new cases were recorded, significantly higher than the 1,813 registered Saturday.
"The rise in the number of cases that we have seen today is concerning," said Health Secretary Matt Hancock as Britain digested the highest number of daily cases since 2,959 on May 23.
Hancock said the latest sufferers are predominantly young people but cautioned against behaviour that might allow a spread to the elderly.
Hancock said it was "important that people don't allow this illness to infect their grandparents and to lead to the sort of problems that we saw earlier in the year" when the health system battled to contain the virus as the toll inexorably rose.
The concern is that increased transmission generally heightens the risk that the virus could get passed on to the elderly and people with underlying health problems.
Opposition Labour health spokesman Jonathan Ashworth urged Hancock to update lawmakers on the government's approach in parliament on Monday.
Prime Minister Boris Johnson's Conservative government says it will tighten local restrictions in areas showing sharp rises in cases rather than impose a second national lockdown for fear of its effect on the economy.
Such restrictions would include limiting household contacts.
Sunday saw 124 new admissions to hospital of virus sufferers, bringing the total to 756 with 69 on ventilators.
- 'Get Britain back up and running' -
To date, a total of 347,152 coronavirus cases have now been confirmed across the United Kingdom.
How to deal with the virus spread just as the school year starts remains a delicate question in many countries. Some parents question if it safe for their offspring to return to the classroom.
Hancock said the British government was right to reopen schools "because of the impact on children of not getting an education".
London's mayor Sadiq Khan expressed concern at the latest "worrying" transmission data.
"Londoners have already made incredible sacrifices but the virus is still with us & we must keep following the rules to keep safe," he tweeted.
Beyond the headline figure for Britain on Sunday, Scotland recorded 208 new cases for its highest daily increase in more than four months."
Cities are struggling with the economic impact of the virus, with millions of people having spent months working from home.
"The economy needs to have people back at work," Foreign Secretary Dominic Raab told the BBC.
"It is important to send a message that we need to get Britain back up and running, the economy motoring on all cylinders."
But he accepted that a mass return to office working would need to be "incremental".