The UK faces an "unquestionably difficult" winter, Boris Johnson admitted after announcing coronavirus restrictions which could last six months.
The British prime minister set out a package of measures for England, with office staff once again working from home, the wider use of face masks and a 10pm curfew on pubs and restaurants.
He said the new curbs could continue well beyond Christmas and the New Year unless there was progress in controlling the spread of the virus.
He also indicated tougher restrictions could be imposed if they failed.
Mr Johnson used a televised address to plead with the public to follow the new rules, claiming that breaches of previous restrictions by "freedom-loving" Britons had helped the virus return.
The new package of measures - and similar or tougher measures elsewhere in the UK - followed scientists' warnings that the number of cases was doubling every seven days.
"While the vast majority have complied with the rules, there have been too many breaches - too many opportunities for our invisible enemy to slip through undetected. The virus has stated to spread again in an exponential way. Infections are up, hospital admissions are climbing," Mr Johnson said.
Businesses will face £10,000 (€10,900) fines or closure for failing to comply with coronavirus rules, and people risk £200 penalties for failing to wear masks or breaching the "rule of six".
The military could be used to free up police officers to tackle coronavirus rulebreakers.
Mr Johnson said he was "deeply, spiritually reluctant" to infringe on people's freedoms, but unless action was taken now there would be a need for harsher measures later "when the deaths have already mounted".
If people do not follow the new rules we have set out then "we must reserve the right to go further".
But Mr Johnson finished his televised address with a plea for a "spirit of togetherness", to get the country though the months ahead. "If we follow these simple rules together we will get through this winter together," he said.
"There are unquestionably difficult months to come. The fight against Covid is by no means over. I have no doubt, however, that there are great days ahead."
Mr Johnson set out measures for England in the House of Commons earlier yesterday, but stressed all parts of the UK were united in the need for decisive action.
In Scotland, first minister Nicola Sturgeon went further, imposing a ban on household visits from today.
Mr Johnson said the UK had reached "a perilous turning point" in the face of a second wave of Covid-19 cases.
The number of confirmed coronavirus cases in the UK passed 400,000, with a further 4,926 lab-confirmed cases as of 9am yesterday.
The British government said a further 37 people had died within 28 days of testing positive for Covid-19 as of Tuesday.
Labour leader 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".
Paul Hunter, professor in medicine at the University of East Anglia, said there was a question over whether the measures announced by Mr Johnson would "be sufficient".