The UK Covid alert level has been raised from level 3 to level 4 following advice from the UK Health Security Agency.
The UK Chief Medical Officers and NHS England National Medical Director have made the recommendation to ministers as early evidence shows that Omicron is spreading much faster than the Delta variant.
A joint statement was issued by Chief Medical Officers of the four nations after the raising of covid levels.
It said: "Following advice from the UK Health Security Agency and in the light of the rapid increase in Omicron cases, the UK Chief Medical Officers and NHS England National Medical Director have recommended to Ministers that the UK COVID Alert Level should increase from Level 3 to Level 4.
"Transmission of COVID-19 is already high in the community, mainly still driven by Delta, but the emergence of Omicron adds additional and rapidly increasing risk to the public and healthcare services.
"Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced.
"Data on severity will become clearer over the coming weeks but hospitalisations from Omicron are already occurring and these are likely to increase rapidly."
The statement was issued on behalf of Chris Whitty, Professor Sir Michael McBride, Professor Gregor Smith Dr Frank Atherton and Professor Stephen Powi.
It urged people to arrange booster vaccines to increase their immune systems in light of the increased risk of catching the virus.
"When vaccine protection is reduced in the way that is happening with Omicron it is essential to top up that protection with a booster.
"Both booster vaccines (Pfizer and Moderna) increase the immune response substantially and show good effectiveness although with some reduction compared to Delta.
"The NHS is currently under pressure mainly driven by non-COVID pressures. With a variant spreading with increased transmissibility and reduced vaccine effectiveness, we are likely to see this pressure rise soon.
"It is extremely important that if you are eligible, you get your COVID vaccination now – whether this be your first, second or booster dose.
"People should continue take sensible precautions including ventilating rooms, using face coverings, testing regularly and isolating when symptomatic."
