The UK's Covid-19 alert level will be raised to the highest level, government sources have said.
It is understood the alert level will be upgraded from Level Four to Level Five, which means there is a risk that NHS services will be overhwhelmed.
The alert level is based on a recommendation by the Joint Biosecurity Centre (JBC) to the chief medical officers.
The UK Chief Medical Officers (CMOs) also released a joint statement recommending the change.
The statement, signed by Chief Medical Officer for England, Professor Chris Whitty; Chief Medical Officer for Wales, Dr Frank Atherton; Chief Medical Officer for Scotland, Dr Gregor Smith; Chief Medical Officer for Northern Ireland, Dr Michael McBride and NHS England, National Medical Director Professor Stephen Powis, said: "Following advice from the Joint Biosecurity Centre and in the light of the most recent data, the 4 UK Chief Medical Officers and NHS England Medical Director recommend that the UK alert level should move from level 4 to level 5.
"Many parts of the health systems in the 4 nations are already under immense pressure. There are currently very high rates of community transmission, with substantial numbers of COVID patients in hospitals and in intensive care.
"Cases are rising almost everywhere, in much of the country driven by the new more transmissible variant. We are not confident that the NHS can handle a further sustained rise in cases and without further action there is a material risk of the NHS in several areas being overwhelmed over the next 21 days.
"Although the NHS is under immense pressure, significant changes have been made so people can still receive lifesaving treatment. It is absolutely critical that people still come forward for emergency care. If you require non-urgent medical attention, please contact your GP or call NHS 111."
Prime minister Boris Johnson will be making a televised address tonight setting out new emergency measures to control the spread of coronavirus in England.
The statement, to be made at 8pm on Monday, will be followed by the recall of Parliament on Wednesday so MPs can debate the measures.
How does the Covid-19 alert system work?
The Covid-19 alert level system was unveiled by the government in May last year.
It is used to indicate how tough coronavirus measures in the UK should be.
Level Five indicates that social distancing measures should be increased, according to the scale.
The alert level is determined by a range of indicators and is informed by public health data and statistical modelling.
For the alert level to change, the JBC's recommendation needs to be reviewed and agreed by the chief medical officers of the four UK nations.
The government is so far yet to confirm the move from Level Four to Level Five.
The threat level is not the same as the tier system, which also uses levels to determine which areas should be placed in which tiers.
