Prof Whitty warned: “There is a reasonable chance that at least there will be some degree of vaccine escape with this variant.”

Face coverings will become mandatory in shops and on public transport once again, after a previous legal requirement was lifted in July, as part of a wider easing of Covid-19 restrictions. Hospitality settings such as bars, pubs and restaurants will be exempt from the rule.

Separately, Mr Johnson said that everyone deemed to be a “contact” of someone who has tested positive for the omicron variant will be told to self-isolate for 10 days, regardless of whether they have been vaccinated. Since August 16, amid a summer easing of Covid-19 restrictions, fully-vaccinated people have not had to self-isolate if a contact tests positive for the virus.

All travellers arriving from abroad will have to self-isolate until they receive a negative result from a PCR test, which they will be required to take on the second day after entering the country.

Sajid Javid to lay out regulations before Parliament

Mr Javid is expected to lay out regulations before Parliament to impose the restrictions. The new measures will initially last for three weeks, after which they will be reviewed.

Mr Johnson will likely face resistance from some Conservative backbenchers to any new legal restrictions. Many will fear that even more draconian rules could follow.

But Sir Patrick said: “The thing that we need to do is always think in the face of this virus, go earlier than you think you want to, harder than you think you want to and more geographically broad than you think you want to.”

Asked whether a nationwide lockdown could be ruled out, Sir Patrick said: “If it’s very transmissible and obviously does cause big escape, then clearly that’s a major issue to face up to, but that isn’t what we know at the moment.”

Mr Johnson urged those already eligible for the third jab to get their booster.

Suggesting that the new suite of measures could help to ensure that Britain avoids the need for further restrictions over Christmas, he said: “As we go forward to Christmas, we are in a strong position but the objective of what we’re doing tonight is to keep that position strong.”

Currently, only those aged 40 and older are eligible for a booster vaccine and ministers have been anxious to see the rollout extended to under 40s as soon as possible.

Mr Johnson said that Mr Javid had asked the JCVI to “consider giving boosters to as wide a group as possible, as well as reducing the gap between your second dose and your booster”.

The Prime Minister heaped public pressure on the JCVI on Saturday night, stating: “They are an independent body but, clearly, we hope that we’ll get some answers for everybody as soon as possible.”

Mr Johnson said that six million jabs would be given to patients in England alone over the next three weeks.

The Czech Republic also identified its first suspected case of omicron in a woman who recently travelled to Namibia. The virus has also been found in Belgium, Israel, Hong Kong and Botswana.