London,  UK Prime Minister Boris Johnson warned that the nation will see a 'tidal wave' of Covid's Omicron variant, even as the country's coronavirus alert level was raised to four due to the spread of the super mutant strain of the lethal virus.

Level four means a high or rising level of transmission -- the last time the UK was at this level was in May.

Declaring an "Omicron emergency", Johnson also set a new booster target -- the third doses will be offered to everyone over 18 in England from this week, three months after their second dose, the BBC reported.

"No one should be in any doubt, there is a tidal wave of Omicron coming," Boris Johnson said on Sunday.

People have been told to work from home from Monday "if they can", as part of the UK government's introduction of Plan B measures.

"I'm afraid we're now facing an emergency in our battle with the new variant Omicron," said Johnson.

"It is now clear that two doses of vaccine are simply not enough to give the level of protection we all need. But the good news is that our scientists are confident that with a third dose, a booster dose, we can all bring our level of protection back up."

"At this point our scientists cannot say that Omicron is less severe.

"And even if that proved to be true, we already know it is so much more transmissible that a wave of Omicron through a population that was not boosted would risk a level of hospitalisation that could overwhelm our NHS and lead sadly to very many deaths."

Johnson noted that to reach the new jab target, certain other medical appointments would need to be postponed to the New Year. Some general practitioners are already allowed to postpone routine health checks to make space for vaccinations, the report said.

Meanwhile, Health Secretary Sajid Javid on Monday, confirmed that 10 people in England are hospitalised with Omicron, with no deaths confirmed.

However, he told BBC Breakfast that 40 per cent of all Covid cases in London were of the variant, which was spreading at a "phenomenal rate".

Asked whether the new measures were excessive, Javid said it was important to act now, given the lag in hospitalisations in deaths that follow a rise in cases - even if Omicron turns out to be milder than other variants.

Early data suggests that getting a third booster dose gives an individual around 70-75 per cent protection against symptomatic infection from Omicron.

More than half a million booster jabs and third doses were given in the UK on Saturday.

A further 1,239 new cases of Omicron were announced in the UK on Sunday, bringing the total number of UK Omicron cases to more than 3,000. The real number is estimated to be much higher, the report said.