The UK Covid alert level has been raised following a rapid increase in omicron cases being recorded.

       The country's four chief medical officers and NHS England's national medical director have recommended to ministers that the UK go up to Level 4 from Level 3.

       Boris Johnson addressed the nation at 8pm on Sunday to warn of a "tidal wave" of omicron, which is more resistant to vaccines, and said from "bitter experience" the situation could deteriorate rapidly.

       "Do not make the mistake of thinking omicron can't hurt you, can't make you and your loved ones seriously ill," he warned, highlighting that hospital admissions in South Africa have doubled within a week.

       He also confirmed the launch of the Omicron Emergency Boost, an initiative to get all adults their third doses before the New Year - amounting to 18 million doses.

       Why have we moved to Level 4?

       The decision to increase the alert level follows advice from the UK Health Security Agency (UKHSA) after a further 1,239 confirmed cases of the omicron mutation were recorded in the UK as of Sunday.

       It brings the total number of UK cases of omicron to 3,137, a 65 per cent increase from Saturday's total of 1,898 UK cases.

       Increasing the UK Covid alert level to Level 4 means the epidemic is "in general circulation, transmission is high and direct Covid-19 pressure on healthcare services is widespread and substantial or rising", according to Government guidance.

       In a joint statement, the CMOs and NHS England's Professor Stephen Powis said the emergence of omicron "adds additional and rapidly increasing risk to the public and healthcare services".

       They added: "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 five officials, including England's CMO Professor Chris Whitty, Northern Ireland's Sir Michael McBride, Scotland's Professor Gregor Smith, and Wales' Dr Frank Atherton, said the NHS was already under pressure "mainly driven by non-Covid pressures", with omicron's ability to escape vaccines "likely" to add to those demands.

       "It is extremely important that if you are eligible, you get your Covid vaccination now - whether this be your first, second or booster dose," they said.

       "People should continue to take sensible precautions including ventilating rooms, using face coverings, testing regularly and isolating when symptomatic."

       Will this mean more restrictions?

       The development in levels comes as it was warned more restrictions may be needed to tackle omicron, with the UK facing an "inevitable" large wave of infections.

       That is according to Dr Susan Hopkins, the chief medical adviser for the UKHSA, who said people will have to reduce social contact as much as possible, with omicron cases being detected among hospital patients - although no deaths have yet been reported as a result of contracting it.

       Dr Hopkins said more measures may be needed, adding that the Government has "very difficult" decisions ahead, even after triggering its winter Plan B proposals, which include introducing Covid passports for large venues.

       "I think that the restrictions that the Government have announced are sensible. I think that we may need to go beyond them. But we'll need to watch carefully what happens with hospitalisations," she told the BBC.

       When was the last time the UK was in Level 4?

       The last time the alert level was Level 4 was in February, when on the 25 Feb the alert level was downgraded from Level 5 to Level 4.

       During that time, the restrictions were as follows:

       Primary and secondary schools were closed. Indoor hospitality and non-essential retail, such as hairdressers and clothes shops, were closed. Entertainment venues, such as cinemas, zoos and theme parks, were closed. ‘Stay At Home’ order was in-place. Organised outdoor sport was banned. Mixing between households was prohibited. Hugs were banned.

       However, no further restrictions than those currently imposed by Plan B have been announced.