The Prime Minister will tell the public everyone must do their bit to help slow the new Omicron variant rapidly spreading across the world. Very little is known about the mutation, but scientists fear it is more vaccine-resistant than other strains of the virus.

       So far 14 cases of Omicron have been found in the UK, five in England and nine in Scotland.

       From today, face coverings are once again compulsory in England in shops and settings such as banks, post offices, hairdressers, and public transport.

       A PCR test is also now required for all travellers returning to the UK from around the world, with people forced to self-isolate until they receive a negative result.

       All contacts of suspected Omicron cases must self-isolate, regardless of their age or vaccination status.

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       Mr Johnson will use his Downing Street address, expected to be at 5pm, to tell the public the new measures are necessary to ensure the success of the UK's vaccination programme is not undermined.

       He will reinforce his message that the extra restrictions are "temporary" and "precautionary" and will be revied in three weeks.

       Some fear even more measures could be introduced at that time if the Omnicron variant is surging through the UK and risks leading to an increase in hospitalisations.

       Scientists are urgently investigating the Omicron variant to understand more about it and just how much of a threat it is.

       It has far more mutations than previous strains of the virus, which has led to concerns the jabs currently being deployed will be less effective.

       Working is already underway to adapt vaccines, with new injections set to be ready in 100 days.

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       Yesterday deputy chief medical officer Professor Jonathan Van-Tam admitted the number of mutations present in the Omicron variant "makes us worry about a possible effect on vaccine effectiveness".

       However, he insisted it was "not all doom and gloom", saying: "I do not want people to panic at this stage.

       "If vaccine effectiveness is reduced - as seems pretty likely, to some extent - the biggest effects are likely to be in preventing infections and, hopefully, there will be smaller effects in preventing severe disease."

       Mr Johnson will tonight also urge Britons to make sure they get their Covid jabs after the Joint Committee on Vaccination and Immunisation (JCVI) have its approval for teenagers to be given a second jab, all adults to be offered a booster injection, and for the gap between the second and third dose to be slashed in half to three months.

       This morning health minister Gillian Keegan told Sky News the Government is "very much hoping that we can keep Christmas on track" with the extra measures it is bringing in.

       She added: "Of course Christmas is on track, and actually what everybody wants for Christmas is if you haven't had your first jab, come and get it, if you haven't had your second jab, come and get it, and if you haven't had your booster, come and get it when you're asked."

       So far more than 17.8 million Britons have received a boost vaccine.

       Meanwhile, 50.9 million, 88.6 percent of the population aged 12 and over, have had at least one dose.