England's deputy chief medical officer said the rise in the number of coronavirus cases was of great concern and people had "relaxed too much" over the summer.
Warning people they would need to follow the guidelines, Jonathan Van-Tam said: "We have got to start taking this very seriously again".
"If we're not careful we're going to have a bumpy ride, people have relaxed too much," he said.
The United Kingdom recorded 2,948 daily confirmed cases of COVID-19, according to government data published on Monday, the second biggest 24-hour rise since May.
Daily case numbers had been rising at about 1,000 a day for most of August, but have started to increase in recent days. Britain's testing capacity has also increased since the peak of the first wave earlier this year.
The United Kingdom has suffered more than 65,000 excess deaths from coronavirus, according to the government's statistics office, with a surge that lasted longer and spread to more places than those in other hard-hit European nations like Italy and Spain.