Households in England's second city, Birmingham, are being urged to stop socialising with others as part of a dramatic tightening of coronavirus restrictions after a spike in new confirmed cases.
While a study on Friday revealed that the epidemic is doubling every seven to eight days in England, the mayor of the West Midlands Andy Street announced the tougher restrictions on household mixing in Birmingham and the nearby boroughs of Solihull and Sandwell.
Though the ban formally comes into effect on Tuesday, Street advised affected residents "to avoid household mixing before then as it has been identified as one of the drivers of transmission".
Street said the restrictions don't apply to schools, workplaces or transportation.
"It is about household mixing," he said.
The move - which affects more than 1 million people - is the latest hardening of restrictions following a recent spike in cases across the UK, particularly among young adults.
Earlier this week, the British government, which has the sole responsibility for public health matters in England, said social gatherings will be limited to six people from Monday both indoors and outdoors and that the new rules will be policed more stringently.
The other parts of the UK - Scotland, Wales and Northern Ireland - have also announced similar restrictions on gatherings.
Over the past week, the number of confirmed cases in the UK has spiked dramatically.
The daily increase has been running at near 3000 for much of this week, around double the number of the previous week.
The Scientific Advisory Group for Emergencies, which advises the government on virus-related matters, said the virus' transmission rate was likely increasing across the whole of the UK.
It said the reproduction rate is now between 1.0 and 1.2, meaning anyone with the virus is infecting on average a little more than one other.
The R number was largely flat or below 1 over the northern summer, meaning the epidemic getting smaller.
Separately, a survey of more than 150,000 volunteers who were tested between August 22 and September 7 by Imperial College London and polling firm Ipsos MORI indicated that cases are increasing steadily across England and that the epidemic was doubling every seven to eight days.
"We've seen all across the world how a rise in cases, initially among younger people, leads to hospitalisations and fatalities," said Health Secretary Matt Hancock.
"The pandemic is not over, and everyone has a role to play to keep the virus at bay and avoid another further restrictions."
The UK has Europe's worst death toll from the virus, with more than 41,600 deaths within 28 days of testing positive.