Boris Johnson announces a new four-week lockdown across England, starting from Thursday
Bars and restaurants will close, but takeaways can continue to operate - as in the spring lockdown
Shielding measures will not be reintroduced, but vulnerable people or those aged over 70 are told to be especially careful
Schools and universities will be kept open throughout this period, unlike in the lockdown earlier this year
The furlough system paying 80% of employee wages will be extended through November
The current surge in the virus has seen the UK's total number of positive cases pass the one million mark - it now stands at 1,011,660
One model, seen by the government, forecasts up to 4,000 deaths a day during the winter months no action is taken
Over the past seven days, the UK has averaged almost 23,000 new cases a day, and 237 deaths with coronavirus
The PM says while scientists are "gloomy" they are "unanimously optimistic about the medium and the long-term future".
"We're not going back to the full-scale lockdown of March and April," he says.
"But I'm afraid from Thursday the basic message is the same: stay at home, protect the NHS and save lives."
The BBC's Laura Kuenssberg asks Johnson about the timing of the decision.
“This is a constant struggle and balance that any government has to make between lives and livelihoods and lives must come first but we have to be mindful of the scarring, the long term economic impact of the measures," he says.
"If we could have got the R down in the local areas in the way we wanted to get it below 1, that would have been a great way forward, in common with other parts of Europe, we’ve seen an overall growth rate in the second wave and made it vital to act now and protect the NHS and save lives."
He says it's true the course of the pandemic has changed and insists it's right the government should "modulate its response in accordance and I make no apologies for that".
The PM says: "I am optimistic that this will feel very different and better by the spring."
He says this is because of the "immediate prospect" of rapid turnaround coronavirus tests.
"Over the next few days and weeks we plan a steady but massive expansion in the deployment of these quick turnaround tests," he says.
They could be used to test "whole towns and even whole cities", says the PM, adding that the programme will "begin in a matter of days" and will be assisted by the military.
Parliament will debate and vote on these measures on Wednesday.
Johnson says the government in Westminster has updated the devolved administrations, and ministers "stand ready to work with them on plans for Christmas and beyond".
He says "we are not alone in what we're going through" - pointing to European countries that have introduced lockdown measures.
Unlike the first national lockdown, schools, colleges and universities will be allowed to stay open.
"We cannot let this virus damage our children's futures even more than it has already," says Johnson.
He urges parents to continue sending their children to school.
Christmas will be "perhaps very different" this year, Johnson says.
"But it's my sincere hope and belief that by taking tough action now, we can allow families across the country to be together," he adds.
The PM says he is under "no illusions" about how difficult it will be for businesses - he says he is "truly sorry" for that.
He is extending the furlough scheme through November, he says.
Johnson confirms the measures will end on 2 December.
He says, at that point: "We'll seek to ease restrictions, going back into the tiered system on a local and a regional basis according to the latest data and trends."
Takeaways will be allowed to stay open but pubs, bars and restaurants will have to close, says the PM.
Essential shops will remain open so "there's no need to stock up", he insists, adding that workplaces should stay open where people cannot work from home.
Support bubbles will still be allowed and children will be able to move homes if their parents are separated.
He says the government will not ask people to shield again "in the same way" as last time.
Johnson outlines the measures that he says will last until the start of December.
And he lists some of the exceptions for which people can leave their homes, including:
"Now is the time to take action because there is no alternative," the PM says.
He says from Thursday people in England "must stay at home".
The prime minister says the overrunning of the NHS would be a "moral disaster".
Doctors and nurses would be "forced to choose which patients to treat... who would live and who would die," he says.
Nick Eardley
Political correspondent
Remember the government desperately wanted to avoid a national lockdown. The PM has been highly critical of Labour for suggesting one. A number of Mr Johnson’s backbenchers are strongly against national measures.
But it seems the Government’s hand has now been forced by the spread of the virus.
The PM says he still wants to take a "pragmatic" and targeted approach.
But he says the virus is spreading "even faster than the reasonable worst case scenario" of scientific advisers.
It's Boris Johnson's turn to speak now and he says "no responsible prime minister can ignore the message of those figures".
He says: "No one wants to be imposing these kind of measures anywhere."
He says he'd hoped that with "strong local action" the government could get the rate of infection down in targeted areas.
Sir Patrick Vallance is speaking now and says some of the measures have eased the rate of growth but infections continue to rise.
He said experts created scenarios on the assumption the R rate stays at one over the course of the winter.
“What is clear from all the scenarios is the potential for this to be twice as bad or more compared to the first wave."
Speaking about people in hospital with Covid, Prof Whitty says: "If we do nothing, the inevitable result will be that these numbers will go up and they will eventually exceed the peak we saw in spring of this year."
Prof Whitty says we have "very little headroom" when it comes to the spread of the virus.
He says the virus doesn't remain constrained to one age group, and it is spreading into older age groups - people over the age of 60.
He adds there is a rise in the number of hospital admissions in England in "virtually every age group in older adults".
England’s chief medical officer Prof Chris Whitty says "virtually across the entire country now" there is a "significant rate of increase" in virus cases.
He says the prevalence of Covid has been going up "extremely rapidly" over the last few weeks" - having been "very flat" due to social distancing measures in the spring and summer.
Boris Johnson is about to start speaking. As on 23 March, this is the moment when millions of people will learn how their lives are about to dramatically change over the next few weeks.
The prime minister is flanked by two familiar faces - England's chief medical officer Prof Chris Whitty and the government's chief scientific adviser Sir Patrick Vallance.