Britain faces a "tidal wave" of infections from the Omicron coronavirus variant, Prime Minister Boris Johnson says, with a huge increase in booster vaccinations needed to strengthen defences against it.

In a televised statement on Sunday, Johnson said everyone age 18 and older will be offered a third shot of vaccine by the end of this month in response to the omicron "emergency".

The previous target was the end of January.

He said cases of the highly transmissible variant are doubling every two to three days and "there is a tidal wave of Omicron coming".

"And I'm afraid it is now clear that two doses of vaccine are simply not enough to give the level of protection we all need," Johnson said.

"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."

The prime minister announced a "national mission" to deliver booster vaccines, with pop-up vaccination centres, seven-day-a-week clinics getting support from teams of military planners, and thousands of volunteer vaccinators.

Johnson's December 31 target applies only to England. The other parts of the UK - Scotland, Wales and Northern Ireland - are also expected to speed up their vaccination campaigns.

UK scientists believe existing vaccines may be less effective in preventing symptomatic infections in people exposed to Omicron, though preliminary data shows effectiveness appears to rise to between 70 and 75 per cent after a third vaccine dose.

Johnson's announcement came hours after the government raised the country's official coronavirus threat level on Sunday, warning the rapid spread of Omicron had pushed the UK into risky territory.

The chief medical officers of England, Scotland, Wales and Northern Ireland said the emergence of the highly transmissible new strain "adds additional and rapidly increasing risk to the public and healthcare services" at a time when COVID-19 is already widespread.

They recommended raising the alert level from three to four on a five-point scale. The top level, five, indicates authorities believe the healthcare system is about to be overwhelmed.

The doctors said early evidence shows Omicron is spreading much faster than the currently dominant Delta variant, and that vaccines offer less protection. British officials say Omicron is likely to replace delta as the dominant strain in the UK within days.

"Data on severity will become clearer over the coming weeks but hospitalisations from Omicron are already occurring and these are likely to increase rapidly," they said.

Concerns about the new variant led Johnson's Conservative government to reintroduce restrictions that were lifted almost six months ago.

Masks must be worn in most indoor settings and people are being urged to work from home if possible.