The UK's coronavirus alert level is being upgraded from 3 to 4, meaning transmission is "high or rising exponentially", its chief medical officers have said.
It comes after the government's scientific adviser warned there could be 50,000 new coronavirus cases a day by mid-October without further action.
On Monday, a further 4,368 daily cases were reported in the UK, up from 3,899.
The prime minister will make a statement in the Commons on Tuesday.
BBC political editor Laura Kuenssberg said it sounded "very likely" that pubs and other venues in England will be forced to have 22:00 closing times, alongside other measures.
In a statement confirming their recommendation on moving to level 4, the chief medical officers for England, Scotland, Wales and Northern Ireland said cases were now "rising rapidly and probably exponentially in significant parts of all four nations".
They urged people to follow government guidelines "to avoid significant excess deaths and exceptional pressure in the NHS" over the autumn and winter.
Health Secretary Matt Hancock said the move reflected "the significant shift in the current threat posed by coronavirus".
"This country now faces a tipping point in its response and it is vital everybody plays their part now to stop the spread of the virus and protect lives," he said.
The alert level, which is recommended by the Joint Biosecurity Centre, was reduced from level 4 to 3 on 19 June - which indicated the virus was "in general circulation" but there could be a "gradual relaxation of restrictions".
The proposed upgrading comes as the PM prepares to chair a Cobra emergency meeting on Tuesday morning - which will be attended by the leaders of Scotland, Wales and Northern Ireland.
Amid data showing London was "catching up" with Covid-19 hotspots in northern England, the capital's Mayor Sadiq Khan said he believed acting early, "rather than having to impose more stringent measures later", was the right thing to do both for public health and the economy.
Speaking at Downing Street earlier, alongside chief medical adviser, Prof Chris Whitty, Sir Patrick Vallance said: "At the moment we think the epidemic is doubling roughly every seven days.
"If, and that's quite a big if, but if that continues unabated, and this grows, doubling every seven days... if that continued you would end up with something like 50,000 cases in the middle of October per day.
"Fifty-thousand cases per day would be expected to lead a month later, so the middle of November say, to 200-plus deaths per day.
"The challenge, therefore, is to make sure the doubling time does not stay at seven days.
"That requires speed, it requires action and it requires enough in order to be able to bring that down."
The move to level 4 should not come as a surprise given the warning from the UK's two most senior pandemic advisers this morning.
Infections are rising - although some experts question whether the situation is as dire as Prof Chris Witty and Sir Patrick Vallance set out when they raised the prospect of 50,000 cases a day by mid-October.
Cases were always expected to increase at this time of year when respiratory viruses tend to circulate more coupled with the continued re-opening of society.
Certainly the trajectory of countries like France and Spain is not as sharp as the worst-case scenario put forward.
But it is clear the government wants to act early this time - one of the big criticisms is that they were slow to introduce lockdown in March, which resulted in more deaths.
Level 4 paves the way for extra restrictions to be introduced with an announcement expected on Tuesday.
Officials are very aware a fine balance needs to be navigated, which is why a full lockdown is not on the cards.
Schools will certainly be protected.
But any restrictions have a cost to society. Go too far and the risk is the cure becomes worse than the disease.
On Sunday, the prime minister held a meeting in Downing Street with Prof Whitty, Chancellor Rishi Sunak and Health Secretary Matt Hancock to discuss possible further measures for England, ahead of an expected announcement on Tuesday.
Shadow health secretary Jon Ashworth said Labour did not want to see another lockdown but it would be understandable if new measures were introduced as the "exponential growth of the virus cannot be ignored".