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.
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.
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.
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.
'Tough calls are still to come'
The new restrictions in England amount to little more than a tinkering around the edges.
Last week there was talk of mini lockdowns and circuit breaks. But with hospital admissions and deaths still low, there is a window of opportunity to see what impact these new measures will have.
While the rapid growth put forward by the government's chief pandemic advisers on Monday that cases could reach 50,000 a day by mid-October is unlikely to be realised, according to many, the chances are infection rates will continue to climb.
This is a virus which can be transmitted silently - people are infectious before symptoms develop, while some do not even develop any - and we are entering the time of year when respiratory illnesses circulate more and deaths start to increase.
If and when this happens, the government will need to decide how far it is prepared to go. The same applies for the rest of the UK nations despite the tighter restrictions there on visiting people's homes.
A full lockdown is highly unlikely, but more significant steps such as closing hospitality venues, leisure facilities and curbing everyday activities from playing sport to travelling around the country remain options.
But the horrible question that will be asked is this: How much economic pain, disruption and damage to wider health and wellbeing is worth the lives that are at risk?
The really tough calls are still to come.
Labour leader Sir Keir Starmer said he supported "necessary" stricter measures, but added that families were worried "the government doesn't have a strategy".
"One day people were encouraged to work in the office, in fact more than encouraged they were openly challenged by the prime minister for not doing so. Today they're told the opposite," he said.
"This is a time of national crisis but we need clear leadership."
Sir Keir also said it would be a "disaster" to withdraw the furlough scheme - which is due to end next month - in "one fell swoop" with no additional support for people whose jobs might be at risk.
The governor of the Bank of England, Andrew Bailey, suggested some sectors may benefit from further targeted help.
The Confederation of British Industry, which represents business, also called for a targeted replacement for the furlough scheme.
"It has saved thousands and thousands of jobs but there is a cliff-edge looming. And, now, with today's announcement that is more urgent than ever," said director general Dame Carolyn Fairbairn.
Some business owners said the latest rules would make life even harder. Marc Gough, who runs a crockery and glassware business in the wedding and events sector, said the cut to the limit for weddings to 15 was "heart breaking".
"I can't earn a living because you're restricting me to weddings of 15 people," Mr Gough told BBC Radio 5 live, adding that the turnover of his business had fallen from £750,000 to £20,000.
"To walk into that warehouse, to put a smile on my face to the staff that I have left, to the clients that I deal with constantly, the brides, it's truly heart breaking," he said.
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