In an interview with Robert Peston for ITV News, Professor John Edmunds - a member of the government's Scientific Advisory Group for Emergencies (SAGE) - warned that coronavirus cases are "increasing exponentially".
Prof Edmunds, from the Faculty of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine, said that although we are all still socially distancing far more than we were before the virus arrived, we have not "hit the sweet spot" that allows more normal economic activity and simultaneous control of the spread of the virus.
Prof Edmunds said that the autumn will be a challenge, because the rate of reproduction of the virus or R rate is above one - when schools and universities are re-opening.
He raised the prospect not just of more local lockdowns but also of renewed national curbs on our freedoms - because opening schools and universities would have "an epidemiological effect" everywhere.
"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 1. And we are already above 1 and we've opened schools. So this is a risky period".
He added: "The epidemic continues to increase and then we have Christmas. And that is very difficult. What is Christmas? Well it's meeting with your family very close. Restaurants and pubs and stuff like that. And it's all high risk. And it's all indoors. Indoors makes a difference."
He continued: "We've been lucky with the weather - and it's turned".
In a wide-ranging interview about the pandemic and its fallout, Prof Edmunds told ITV News Political Editor Robert Peston that visiting people in homes "is quite risky".
"You are there for quite a long time. It is impossible to socially distance with family members, things like that," he said.
"That is now having an effect. And we can see the epidemic is taking off again. So I don't think we've hit that sweet spot where we've been able to control the epidemic and allow the economy to return to some sort of normality. I don't think we've managed that."
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On the return of schools across the UK, the scientist said: "I'm quite worried that schools have just returned, cases are increasing now exponentially. They are increasing exponentially now, From a relatively low level. But they are increasing."
"Schools have gone back. Universities are high risk. They are going back in the next couple of weeks. And the government is trying to get us back into work and back on the tube and buying our sandwich from Pret a Manger, and things like that.
"That will have an epidemiological effect," he told ITV News.
"There is a degree of caution. We have not returned to what we were doing before. One of the things I do, we measure people's contact behaviour. And it has not returned to anywhere what it used to be before March. But it is returning and it does carry a risk".
With local lockdowns in force in various areas of the UK, Prof Edmunds was quizzed on the chance of more of them coming into force.
He told Robert Peston: "Local lockdown is a vague term. We undoubtedly will see more measures. I hope that we can do it in a local way. But I don't think that is a guarantee."
"Many of these measures we are now taking, like opening schools and universities, are going to have an effect across the country. Many towns and cities have a university or Further Education college. And every village has a school. And that will have an epidemiological effect throughout the country."
The scientist talked of his fears in balancing easing lockdown restrictions against keeping the reproduction number down.
"My fear over the last few months [...] 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 1. And we are already above 1 and we've opened schools.
He described the time we are in as "A risky period."