The United Kingdom reported 293 deaths from COVID-19 on Tuesday, the highest daily figure since March, and there have been an average of about 40,000 new cases each day in recent weeks.
Prime Minister Boris Johnson lifted restrictions in England in July and has said he plans to cope with COVID-19 over the northern hemisphere winter by relying on vaccinations rather than mandating masks or lockdowns.
"Too many people believe that this pandemic is now over. I personally feel there are some hard months to come in the winter and it is not over," Jonathan Van-Tam told BBC TV, adding that behaviour and uptake of booster shots would determine the severity.
"Christmas and indeed all of the darker winter months are potentially going to be problematic."
Johnson has resisted calls from some scientists to move to a "Plan B" of mask mandates, vaccine passes and some work-from-home orders.
His spokesman pointed to a recent decline in cases following a half-term holiday for schools.
"We don't see anything in the data that suggests a move to Plan B is necessary," Johnson's spokesman said.
"We are by no means complacent and recognise that cases remain at a high level and the picture in the NHS (health service) is extremely challenging."
The Scientific Advisory Group for Emergencies (SAGE) has said that taking action now could reduce the need for tougher measures later.
Jeremy Farrar, director of Wellcome, said he had stepped down from SAGE to focus on his work at the health charity but added the situation was still worrying.
"The COVID-19 crisis is a long way from over," he said in a statement.
"The high levels of transmission seen in the UK remain concerning, but I stepped down as a participant of SAGE knowing ministers had been provided with most of the key science advice needed over the winter months."
