The UK has reached "a perilous turning point", Prime Minister Boris Johnson has said, as he set out a raft of new coronavirus restrictions for England which could last for up to six months.
Shop staff will have to wear face masks and weddings will be limited to a maximum of 15 people, under the rules.
Fines for breaking laws on gatherings and not wearing a mask will increase to £200 for a first offence.
He also warned "significantly greater restrictions" could come if necessary.
Mr Johnson said "similar steps" would be taken across the UK after he met with the leaders of Scotland, Wales and Northern Ireland on Tuesday morning.
Like England, Scotland's First Minister, Nicola Sturgeon, also announced a 22:00 curfew for hospitality businesses. The same measure is also set to come into force in Wales on Thursday.
But Ms Sturgeon went further than England on restrictions, banning the visiting of other households indoors.
It comes as the number of UK cases rose by 4,926 on Tuesday, government figures showed, with deaths increasing by 37.
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Weddings: 'We have to phone 15 people and un-invite them'
Speaking in the House of Common, the prime minister told MPs: "We always knew that while we might have driven the virus into retreat, the prospect of a second wave was real. I'm sorry to say that, as in Spain and France and many other countries, we've reached a perilous turning point."
Mr Johnson said the new rules were "carefully judged" to achieve the maximum reduction in the R number - which measures how quickly the virus is spreading - while causing "the minimum damage to lives and livelihoods".
But he said this was "by no means a return to the full lockdown of March", with no general instructions to stay at home. Businesses, schools, colleges and universities will remain open.
Attendance figures show the number of schools in England sending home groups of pupils due to Covid-19 incidents quadrupled in a week
In the last fortnight, hospital admissions have doubled and Covid-19 is likely to spread faster in winter, Mr Johnson said, adding: "So this is the moment when we must act."
If these restrictions fail to bring the R number below one - the point where the epidemic is no longer growing - "then we reserve the right to deploy greater fire power with significantly greater restrictions" he said.
The latest R estimate for the whole of the UK is between 1.1 and 1.4.
Mr Johnson said: "We will spare no effort in developing vaccines, treatments, new forms of mass-testing but unless we palpably make progress we should assume that the restrictions that I have announced will remain in place for perhaps six months."
Under the new measures for England:
Office workers are being told to work from home again if possible
Penalties for not wearing a mask or gathering in groups of more than six will increase to £200 on the first offence
From Thursday 24 September, all pubs, bars and restaurants will be restricted to table service only. Takeaways can continue
Also from Thursday, hospitality venues must close at 22:00 - which means shutting then, not calling for last orders (in Scotland the same curfew rule comes into force on Friday)
Face coverings must be worn by taxi drivers and passengers from Wednesday
Retail staff and customers in indoor hospitality venues will also have to wear masks from Thursday, except when seated at a table to eat or drink
From Monday 28 September, only 15 people will be able to attend weddings and civil partnerships, in groups of six. Funerals can still take place with up to 30 people
Also from 28 September, you can only play indoor adult sports in groups of less than six
The planned return of spectators to sports venues will now not go ahead from 1 October
Mr Johnson also said the government would provide police and local authorities in England with extra funding to enforce the regulations, with the option to draw on military support.
Rules 'under review'
Restating her support for people working from home, Scotland's first minister also announced her government might impose a "legal duty" on employers to allow home working.
Ms Sturgeon said enforcement of coronavirus rules for the public in Scotland was "under review", but added: "Supporting people to do the right thing is much more effective than threatening harsh punishment if they cannot."
The first minister also said the new restrictions on visiting other households would be reviewed every three weeks - and stressed that they would "not necessarily" be in place for as long as six months.