CNN contacted the Downing Street press office, which confirmed Monday's news conference but said the national lockdown reports were speculation.
In some of the most onerous restrictions in Britain's peacetime history, people will only be allowed to leave home for specific reasons such as education, work, exercise, shopping for essentials and medicines or caring for the vulnerable.
Under the new measures, which will be effective until December 2, people in England will only be allowed to leave their homes for specific reasons, such as education, work or food shopping, said Johnson. "I'm afraid non-essential shops, leisure and entertainment venues will all be closed, though, click and collect services, will continue and essential shops will remain open".
Prof Neil Ferguson, whose modelling was crucial to the decision to impose the first lockdown, said keeping universities and schools open meant infections would decrease more slowly this time.
He added that Christmas will be "different this year" but hopes that by "taking tough action, we can allow families across the country to be together".
Prime Minister Giuseppe Conte would prefer to wait a few days before adding new restrictions, but he is under pressure from factions in his coalition advocating more action as the virus threatens to overwhelm the health system, it said.
"We're not going back to the full-scale lockdown of March and April", he said. It comes as documents suggested the United Kingdom was on course for a much higher death toll than during the first wave.
People in a pub listen to Boris Johnson's announcement.
Kim Kardashian posts more photos of her birthday celebrations amid controversy
It concluded: "You are the most, most, most, most wonderful mother to your four lovely children, and they are flawless ". A hologram of Whitney Houston toured the United Kingdom earlier this year, but the experience received poor reviews.
One of the documents, circulated among members of the government's Sage committee in recent days and discussed by Boris Johnson and Chancellor Rishi Sunak on Friday, says that "all models suggest a significantly higher peak than those estimated at any point in the current Reasonable Worst Case Scenario".
Earlier Saturday, official figures showed that another 21,915 people in Britain have tested positive for COVID-19, bringing the total number of coronavirus cases in the country to 1,011,660.
During his speech, the Prime Minister also confirmed that the furlough or Job Retention Scheme (JRS) would be extended for the duration of the new lockdown.
Mr Johnson said "no responsible Prime Minister can ignore" the rising rates of Covid-19 infections, as he acknowledged that cases had not come down fast enough in the worst affected areas.
That was caused in particular by a halt to construction work, while this time building sites and stores selling building materials will remain open, as well as government services handling permits.
At the Downing Street briefing, Johnson was joined by England's Chief Medical Officer, Chris Whitty, and the government's Chief Scientific Adviser, Sir Patrick Vallance.
Hospitalisations skyrocketed in more than a dozen countries in Europe, with admissions soaring beyond the peak reached last spring in a swathe from Austria to Portugal.