In a statement to the Commons this lunchtime, the Prime Minister unveiled a raft of measures designed to stem the rebounding virus, ahead of his address to the nation at 8pm.
"This is the moment where we must act," he said. "We've reached a perilous turning point."
He warned "drastic" restrictions such as a return to full lockdown would be "inevitable" if the new measures fail to reduce the R number - the rate of reproduction - below 1.
Here's what you missed.
What did Boris Johnson say in his announcement?
The PM announced that staff and customers in the hospitality industry will have to wear face coverings unless they're seated at a table, with fines for refusing to wear masks doubling.
The "rule of six" was also slightly tightened to reduce weddings to 15 in size, with funerals staying at 30.
Plans for the Premier League to welcome back spectators in October were also axed.
In a major U-turn on his message only three weeks ago, the PM also urged people to work from home where it does not hurt businesses.
Mr Johnson also said troops may be drafted in to help support police in enforcing the new rules, with the Ministry of Defence (MoD) saying 500 personnel are on standby to backfill armed guarding duties.
What are the new lockdown measures?
The Prime Minister announced:
- Face coverings will be mandatory for staff in retail, hospitality, and for drivers of taxis and private hire vehicles
- Customers in indoor hospitality will also have to wear face coverings – except while seated at a table to eat or drink
- People should work from home if they can - where not possible they should continue attending workplaces
- Pubs, bars, restaurants must close by 10pm from Thursday - full closure not last orders and must operate table service only
- Businesses will be fined and could be closed for breaching "Covid-secure" guidelines
- A maximum of 15 people will be allowed in wedding ceremonies and receptions, and a maximum of 30 for funerals. The rule of six is extending to all adult team sports
- Business conferences, exhibitions and spectators at sporting events such as the Premier League will not be allowed, as planned, from 1 October
- The exemptions to the rule of six will be reduced, banning indoor team sport – such as indoor five-a-side football matches
- The first offence fine for failing to wear a face covering will double to £200
- More police patrols to enforce restrictions and the military will be drafted for support where needed.
However, the “rule of six” ban on social gatherings indoors or outdoors of more than six people was not tightened to ban households mixing, as in Northern Ireland.
There was also no requirement for people to begin shielding again, except for in local lockdown areas.
The new rules apply only to England. In Scotland, First Minister Nicola Sturgeon announced that households would be banned from mixing indoors from Friday, while Northern Ireland will enforce the same measure from Tuesday at 6pm.
In Scotland, there will be exceptions for those living alone or alone with children, who form extended households, including couples who live apart.
As more measures are announced, the Government will also still be hammering home its “hands, face, space” message as the most effective form of virus control.
Some 13.5 million people across the North West, North East, Wales, Birmingham, the Midlands, and parts of Scotland and Northern Ireland are already in local lockdown.
When do the changes come into place?
The new curfew and table service order for pubs will come into force on Thursday. It is unclear yet when the other measures will come into force.
Mr Johnson said we should "assume" that the curbs will last six months.
He said the new crackdown was "by no means a return to the full lockdown of March, we’re not issuing a general instruction to stay at home".
But in a stark warning of a second lockdown, he added: "I must emphasise, that if all our actions fail to bring the R [number] below 1, then we reserve the right to deploy greater firepower with significantly greater restrictions.
"I firmly want to avoid taking this step, as do the devolved administrations, but we will only be able to avoid it if our new measures work and if our behaviour changes."
In a clampdown on those defying the rules, he added: “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."
Why are the changes needed?
The Cobra emergency committee met on Tuesday morning - only ever convened at the junctures of major crises - with devolved leaders in Scotland, Wales and Northern Ireland all dialling in to discuss the road ahead.
It comes after the UK’s four chief medical officers moved the Covid-19 alert level from level 3 to 4, the second highest threat marker, meaning the “epidemic is in general circulation; transmission is high or rising exponentially”.
Chief scientific adviser Sir Patrick Vallance warned we could be facing 50,000 new coronavirus cases a day by mid-October and a daily death toll of 200 by mid-November "if we do not change course”.
Sir Patrick Vallance, speaking alongside Professor Chris Whitty at a Downing Street briefing on Monday, said the “vast majority of the population remain susceptible” to catching coronavirus and the current situation required swift action to bring the case numbers down.
The UK’s critical R number - the reproduction rate of the virus - has risen to 1.1-1.4, meaning every infected 10 infected people transmit the virus to between 11 and 14 others.
Another 4,368 cases were confirmed on Monday, with another 11 deaths, bringing the UK total to nearly 400,000 cases and more than 41,000 deaths within 28 days of a positive test.