Boris Johnson warned new coronavirus restrictions could last six months, with office staff working from home, the wider use of face masks and a 10pm curfew on pubs and restaurants.
He said that the new curbs could continue well beyond Christmas and New Year "unless we palpably make progress" in controlling the spread of the virus.
The Prime Minister also announced tougher enforcement measures, with businesses facing fines or closure for failing to comply with coronavirus rules, and people facing £200 penalties for failing to wear masks where required or breaching the so-called "rule of six".
The military could be brought in to free up police officers to tackle coronavirus rulebreakers, the Prime Minister said.
He set out measures for England in the House of Commons, but stressed that all parts of the UK were united in the need for decisive action to prevent the virus getting out of control.
In Scotland, First Minister Nicola Sturgeon went further, imposing a ban on household visits from Wednesday.
And she suggested the tougher measures may mean they do not have to be in place as long as restrictions in England.
Mr Johnson said the UK had now reached "a perilous turning point" in the face of a second wave of Covid-19 cases.
His announcement came as the number of confirmed coronavirus cases in the UK passed 400,000, with a further 4,926 lab-confirmed cases as of 9am on Tuesday.
The Government said a further 37 people had died within 28 days of testing positive for Covid-19 as of Tuesday.
The measures being introduced in England over the coming days include:
– Asking office workers who can, to work from home, although construction workers, retail staff and people performing essential services should continue to attend their workplaces.
– From Thursday, pubs, bars and restaurants will be table-service only and hospitality, leisure and entertainment venues will be subject to a 10pm closing time. Takeaways will also close from 10pm to 5am, although they will be allowed to deliver.
– Face coverings will be required for taxi passengers from Wednesday and for retail staff and hospitality customers, except when sitting down eating or drinking, from Thursday.
– Covid-secure guidelines will become legal obligations for retail, leisure and tourism firms, with businesses facing the risk of fines or closure for failing to comply.
– From Monday, a maximum of 15 people will be allowed to attend wedding ceremonies and receptions.
– From Thursday the rule of six will be extended to cover indoor team sports, such as five-a-side football games.
– Plans to allow business conferences and crowds at sporting events from October 1 have been shelved.
Setting out tougher penalties for those who "brazenly defy" the restrictions, Mr Johnson said £10,000 fines would be applied to businesses breaking the rules.
The penalty for failing to wear a mask or breaking the rule of six will double to £200 for a first offence.
The police could be given military support to enable them to focus on coronavirus cases, with troops performing office roles or guarding sensitive sites.
The Prime Minister insisted the measures did not amount to a second lockdown.
"This is by no means a return to the full lockdown of March, we're not issuing a general instruction to stay at home, we will ensure that schools, colleges and universities stay open, because nothing is more important than the education, health and wellbeing of our young people," he said.
But he warned that tougher measures may be needed if the R number, the average number of people someone with Covid-19 infects, remains above one.
"I must emphasise that if all our actions fail to bring the R below one then we reserve the right to deploy greater firepower with significantly greater restrictions," he warned.
Labour leader Sir Keir Starmer said families were worried that "the Government doesn't have a strategy" and "this is a time of national crisis but we need clear leadership".
Mr Johnson was also challenged about the troubled NHS Test and Trace system, run by Tory peer Baroness Harding, but insisted it was "nonsense" to blame that for the rising number of cases.
"Testing and tracing has very little or nothing to do with the spread or the transmission of the disease," he said.
If you live in England, you MUST NOT meet socially in groups of more than 6 people.
You should continue to maintain social distancing with anyone you do not live with.
— Department of Health and Social Care (@DHSCgovuk) September 22, 2020
Paul Hunter, professor in medicine at the University of East Anglia, said there was a question over whether the measures announced by the Prime Minister would "in themselves be sufficient to reverse the increase in cases as we move into autumn".
Prof Hunter added: "It is doubtful that the measures currently being enacted will be sufficient to reduce the R value to below one much before this side of Christmas."
The Government faced renewed calls to do more to support businesses, with the hospitality industry warning that the new restrictions would be a "crushing blow".
Kate Nicholls, chief executive of trade body UKHospitality, said: "It is hard to understand how these measures are the solution to fighting the disease when Government data shows that just 5% of infections out of the home are related to hospitality."
Michael Kill, chief executive of the Night Time Industries Association, warned the measures could trigger "a surge of unregulated events and house parties which are the real hot-beds of infection, attended by frustrated young people denied access to safe and legitimate night-time hospitality venues".
Up to 6,000 jobs are being axed at Premier Inn owner Whitbread, which also operates the Beefeater pubs and Brewers Fayre chains.
The Wetherspoon pub chain also said it had written to its 1,000 airport staff to warn them that between 400 and 450 jobs are at risk of redundancy.