Prime Minister Boris Johnson has urged the public to find the “discipline and resolve” to avoid a second UK lockdown.
In a televised address to the nation, Mr Johnson said there would be "difficult months ahead" but he had hopes of a vaccine and mass testing.
He said the struggle against Covid was the "single biggest crisis" the world had faced in his lifetime - and warned of “many more deaths” if action was not taken.
Mr Johnson said that while the vast majority of people had complied with the rules there had been "too many breaches" and the virus had started to spread again in an "exponential way".
"We know this virus is no less fatal than it was in the spring... we risk many more deaths and many more families losing loved ones before their time," he said.
"The single greatest weapon we bring to this fight is the common sense of the people themselves."
The announcement to the public follows a package of new measures announced by the Government to stop the spread of the virus.
People are now being told to work from home if they can, while restaurants, pubs and bars in England will need to close at 10pm from Thursday.
Meanwhile in Wales, First Minister Mark Drakeford announced the same rule for pubs.
Other measures in England include a new rule that face masks must now be worn by people working in retail, taxis and private hire drivers, and customers and staff in hospitality.
People will also be fined £200 for not wearing a face mask (for their first offence) and from Monday, only 15 people can attend weddings but 30 can attend a funeral.
Mr Johnson warned if people break the rules then the Government would "reserve the right to go further".
"Never in our history has our collective destiny and our collective health depended so completely on our individual behaviour," he said.
"If we follow these simple rules together, we will get through this winter together. There are unquestionably difficult months to come.
"And the fight against Covid is by no means over. I have no doubt, however, that there are great days ahead," he added.
The prime minister's TV address was his third to the nation since the beginning of the coronavirus crisis.
