Boris Johnson warned the UK was at a "perilous turning point" in the battle with coronavirus as he abandoned efforts to get more workers back into offices, ordered wider use of face masks and imposed a 10pm curfew on pubs and restaurants.
And he warned that the new curbs could last for six months – taking them well beyond Christmas – "unless we palpably make progress".
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".
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.
Mr Johnson 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 have reached a perilous turning point."
Measures set out by Mr Johnson to limit the spread of the virus included:
– 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 venues will be subject to a 10pm closing time.
– Face coverings will be required for retail staff, taxi passengers and hospitality customers except where seated.
– 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.
– 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 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.
He indicated troops could be brought in to free up the police to focus on enforcing coronavirus rules.
"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," he said.
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 virus continues to spread with the R number remaining above one.
"I must emphasise that if all our actions failed to bring the R below one then we reserve the right to deploy greater firepower with significantly greater restrictions," he warned.
The Prime Minister said the Government was taking "decisive and appropriate steps to balance saving lives with protecting jobs and livelihoods".
But he acknowledged "this will have profound consequences" for people across the country.
In an effort to quell parliamentary anger – including from some senior Tories – about the way MPs have been sidelined during the crisis, Mr Johnson promised regular statements and the ability to question the Government's scientific advisers.
"No British government would wish to stifle our freedoms in the ways that we have found necessary this year," he said.
"Yet even now we can draw some comfort from the fact that schools and universities and places of worship are staying open, shops can serve their customers,construction workers can go to building sites, and the vast majority of the UK economy can continue moving forwards."