Face masks will be mandatory in shops and on public transport, contacts of Omicron Covid cases must self-isolate, and international arrivals will have to quarantine until they test negative after two cases of the concerning new variant were detected in the UK.

Boris Johnson warned the strain could reduce the effectiveness of vaccines as he announced a strengthening of England’s rules at a Downing Street press conference after the infections were identified in Nottingham and Brentwood, Essex.

The Prime Minister said the “temporary and precautionary” measures will be reviewed in three weeks, while the Government’s vaccine experts will be tasked with considering whether to extend booster jabs to all over-18s.

The UK Health Security Agency confirmed the two cases, which are believed to be connected and linked to travel to southern Africa, after genomic sequencing overnight.

The individuals and their households were ordered into self-isolation and targeted testing is being carried out in areas where they are thought to have been infectious.

Malawi, Mozambique, Zambia and Angola will face travel restrictions from Sunday, when they will join South Africa and five other neighbouring nations on England’s red list.

Mr Johnson said details of the “tightening up” of the mask rules will be outlined by Health Secretary Sajid Javid “in the next day or so”.

But Downing Street said they will become mandatory in shops and on public transport “from next week”, though they will not be required in pubs and restaurant.

To slow the “seeding” of the virus in the UK, Mr Johnson said anyone who enters the UK must now take a PCR test by the end of the second day of their arrival and self-isolate until they receive a negative result.

But the Prime Minister said border measures can “only ever minimise and delay the arrival of a new variant rather than stop it all together”, so all contacts with a suspected case of the new variant will have to isolate for 10 days, regardless of their vaccination status.

Mr Johnson said the measures will be reviewed “in three weeks”, adding: “At that point we should have much greater information about the continuing effectiveness of our vaccines.”

Wary that many on the Conservative backbenches will be angered by the move, Mr Johnson said: “I very much hope that we will find that we continue to be in a strong position and we can lift these measures again, but right now this is the responsible course of action to slow down the seeding and the spread of this new variant and to maximise our defences so that we protect the gains we’ve worked for so hard.”

While the effectiveness of vaccines against Omicron is currently unclear, the Prime Minister said there are “good reasons for believing they will provide at least some measure of protection”.

Mr Johnson stopped short of bringing back the work from home guidance or extending the use of vaccine passports.

England’s chief medical officer Professor Chris Whitty said the Joint Committee on Vaccination and Immunisation (JCVI) will be tasked with looking at whether boosters can be extended down from the over-40s to the over-18s.

The experts will also consider whether second doses should be offered to 12 to 15-year-olds, and the wait before a booster jab could also be reduced.

Mr Johnson said he is “confident” Christmas “will be considerably better than the last”, which was largely cancelled for millions of people, but he refused to say whether further restrictions could be imposed.