The alert level for coronavirus has increased for the first time since June after a review by the countries top doctors.
The chief medical officers from all four UK nations have said the health threat level should increase from three to four.
It means transmission of the virus is "high or rising exponentially".
In a statement from the Joint Biosecurity Committee the experts 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 concerning news for many people; please follow the rules, look after each other and together we will get through this."
Scotland's chief medical officer Dr Gregor Smith is a member of the committee alongside Professor Chris Witty, who issued a stark warning in a TV broadcast today alongside chief scientist Sir Patrick Vallance.
Vallance warned the UK could see 50,000 new coronavirus infections every day by mid-October.
He said this would translate to around 200 deaths each day if the rise was allowed to go unchecked.
Later this week Nicola Sturgeon is expected to announce a "package of restrictions" designed to curb the spread of the virus.
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Although it has been dubbed a second lockdown, the First Minister used her daily briefing to stress any restrictions would be introduced with the aim of avoiding a complete lockdown at a later point.
