Video report by ITV News Political Editor Robert Peston
New lockdown restrictions in England - which include a curfew on pubs and a tightening of the 'rule of six' - are likely to be in place for six months, the prime minister has announced.
Mr Johnson told MPs that the UK has reached a "perilous turning point" in its fight against coronavirus and warned that daily deaths will increase unless action is taken.
He warned that "your harmless cough could be someone else's death knell".
In a statement to the House of Commons, he said the new rules are likely to be in place for six months unless they result in a swift decline in virus infections.
ITV News Political Editor Robert Peston explains what impact the new rules could have
If significant progress is not made on vaccines, treatments or tests, "we should assume that the restrictions that I have announced will remain in place for perhaps six months", Mr Johnson said.
"For the time being, this virus is a fact of our lives and I must tell the House and the country that our fight against it will continue."
From Thursday, all hospitality, leisure, entertainment and tourism businesses will have to shut from 10pm til 5am. They will also be restricted to table service only.
ITV News Political Reporter Shehab Khan and Correspondent Rupert Evelyn analyse the latest developments
Mr Johnson said people should go back to working from home, if they can do so effectively.
The prime minister insisted the new changes are "by no means a return to the full lockdown of March".
He said people are not being urged to stay at home, education settings will remain open, and businesses are being allowed to continue operating, so long as they are Covid-secure.
And those considered extremely clinically vulnerable to coronavirus are not being advised to shield, unless they live in areas of local lockdown.
But the punishments for those in breach of lockdown rules have become much more harsh than they were in the first lockdown.
The penalty for failing to wear a face covering or breaking the rule of six has doubled to £200 for a first offence.
Businesses will also be fined and could be forced to close if they are found to have breached the new rules.
On enforcement, Mr Johnson said: "We will provide the police and local authorities with the extra funding they need, a greater police presence on our streets, and the option to draw on military support where required to free up the police."
The military would fill certain duties, such as office roles and guarding protected sites, freeing up time so police officers can be out enforcing the virus response, a government spokesman said.
The military has not been provided any extra powers, nor will it replace the police in enforcement roles, and they will not hand out fines.
The prime minister also reminded people to continue following social distancing rules, warning: "Your harmless cough could be someone else's death knell."
The 10pm curfew on businesses applies to takeaways, but delivery services will be allowed to continue operating in their usual hours.
Theatres and cinemas will be exempt from the curfew, Culture Secretary Oliver Dowden said.
On work from home advice, which is effective immediately, those working in essential services should continue attending their workplace, if possible, the PM said.
People working in retail, those travelling in taxis, and staff and customers in indoor hospitality - except while seated at a table to eat or drink - would have to wear face coverings.
And no more than 15 people will be able to attend wedding ceremonies and receptions, in a tightening of the "rule of six", which is also effective from Monday.
The tightening will also see indoor sports restricted to just six people, meaning, for example, that indoor five-aside football will be banned.
Labour leader Sir Keir Starmer said it is "right" for Mr Johnson to announce further restrictions, adding: "We support those measures."
Mr Johnson said the government must "act now to avoid still graver consequences later on".
He said: "We always knew that while we might have driven the virus into retreat, the prospect of a second wave was real and I'm sorry to say that, as in Spain and France and many other countries, we've reached a perilous turning point."
A month ago an average of around 1,000 people across the UK were testing positive for Covid-19 every day, Mr Johnson said, adding the latest figure has almost quadrupled to 3,929.
Ahead of addressing MPs the PM chaired Cabinet and the Cobra emergency committee which was also attended by the leaders of Scotland, Wales and Northern Ireland.
The prime minister's official spokesman said the four nations of the UK would broadly follow the same restrictions, but first minister Nicola Sturgeon has gone further with Scotland's new restrictions by banning different households from meeting each other in people's homes.
She said data showed this measure slowed the spread of the virus in the west of Scotland and she hopes by extending it to the rest of the country, it will reduce the rate of transmission.
This measure is in addition to the 10pm curfew for hospitality businesses.
Ms Sturgeon said: “This decision today means we can reduce the amount of time people are able to spend in licensed premises, thereby curtailing the spread of the virus while still allowing businesses to trade and provide jobs.”
“This is the best balance for now.”
Meanwhile the new changes in England had been "carefully judged to achieve the maximum reduction in the R number with the minimum damage to lives and livelihoods", Mr Johnson said.
Watch Boris Johnson's statement to MPs in full: