The UK's coronavirus alert level should rise from three to four, chief medical officers have said.
This means the transmission of Covid-19 is high or rising exponentially, and enforced social distancing should be introduced.
Boris Johnson revealed the alert system in May, before lifting restrictions.
A Level 4 threat could see tougher restrictions introduced to halt transmissions.
The Prime Minster has already warned today (September 21) pubs and restaurants could close later this week.
He suggested a two-week national lockdown could act as a "circuit breaker" against Covid-19, Mirror Online reports.
In a statement, the chief medical officers of England, Scotland, Wales and Northern Ireland said: "After a period of lower COVID cases and deaths, the number of cases are now rising rapidly and probably exponentially in significant parts of all four nations.
"If we are to avoid significant excess deaths and exceptional pressure in the NHS and other health services over the autumn and winter everyone has to follow the social distancing guidance, wear face coverings correctly and wash their hands regularly.
"We know this will be a concerning news for many people; please follow the rules, look after each other and together we will get through this."
Scientists warned Mr Johnson of hundreds of daily coronavirus deaths "within weeks", giving "no alternative" to stringent new coronavirus restrictions.
Today (Monday), a total of 4,368 new cases were confirmed. This brings our nation's total to 398,625.
Sir Patrick Vallance, the chief scientific adviser, said that without interventions, the UK could see 200 or more deaths per day by mid-November.
