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.

This system is separate and independent from any government decisions on easing or tightening rules.

It comes as Boris Johnson prepares to address the nation at 8pm about the booster vaccine programme.

The message - which PA news agency understands will not see new restrictions announced - will be pre-recorded.

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% 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."