England's deputy chief medical officer has warned that Covid is "running too hot" and that there are difficult months ahead.
Professor Jonathan Van-Tam said too many people think the pandemic is behind us, despite very high case rates.
He warned the every day actions of Brits will be crucial in determining how bad the pandemic gets.
Britain has had among the worst infection rates in Europe over the past month, and few restrictions to stop Covid's spread.
So far the Government has resisted calls to launch its Plan B, which would see a return to home working and mandatory face mask wearing.
"The rates are still very high at the moment," Professor Van-Tam told BBC Breakfast and BBC Radio 5 Live.
"They are higher than in most of Europe. And we are running quite hot - I think that's the right expression.
"And, of course, it's of concern to scientists that we are running this hot this early in the autumn season.
"And so, from that perspective, I'm afraid it's caution, followed by caution, and we need to watch these data very carefully indeed over the next days and weeks."
He added: "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.
"The caution that people take or don't take in terms of interacting with each other: that is going to be a big determinant in what happens between now and the darkest months of the winter."
Despite a recent 10% dip in weekly cases, the UK remains at the top of the European charts when it comes to daily coronavirus infections.
The UK also has the fourth highest death rate in the continent, even though the country's vaccination programme is among the most wide reaching.
Over the past week 149 people have died from Covid-19 each day on average.
While the figures are high and the NHS under increasing pressure as winter draws in, the Prime Minister remains confident further measures to restrict the virus's spread will not be needed.
“I see no evidence whatever to think that any kind of lockdown is on the cards," Boris Johnson said last week.
“Absolutely everybody” agrees there is no need to move to tougher curbs despite high infection rates, except “possibly the Labour Party ", he added.
Professor Van-Tam's intervention this morning struck a far more cautious note.
While he did not advocate for a change in government policy, he urged people to get booster shots and to wear masks in certain situations.
The scientist also advised people to wear face masks when not seated at pubs and restaurants, as well as in cinemas and theatres.
Professor Van-Tam also sounded a word of warning over the flu and the extra burden it may place on the NHS.
"Don't forget flu," he said.
"We have very few restrictions in society at the moment - this means all the other respiratory winter viruses will come back.
"We didn't have any flu to speak of last winter because of the lockdowns and so forth, and that means we have a population who are more susceptible, less immune to flu at the moment.
"So, for the people who are at high risk who need a flu jab in the winter, it could never have been more important than right now that you come forward to get it."
Asked if the Government is listening to everything that scientists are putting to it at the moment, Prof Van-Tam said: "The Government has to listen to scientists, it has to listen to all sorts of advice from other places, and it has to make really very difficult decisions about what the best course of action is.
"Science is really important, so is the economy, so is the functioning of society, so these are just tricky decisions, there are no easy choices left and, frankly, there haven't been for a couple of years now."
