Boris Johnson addressed the nation this afternoon, having announced the return of face masks in some settings on Saturday.
The new rules, which also include the return of PCR tests for travellers arriving in the UK from abroad, came into force at 4am today.
It is now mandatory to wear face masks in shops, hairdressers, the post office, banks and on public transport.
However they are not required in hospitality settings like pubs or restaurants at this time in England.
Arrivals to the UK will have to self-isolate, even if fully vaccinated, until they receive the results of a Day 2 PCR test.
The measures have been introduced following the discovery of the new Omicron variant of coronavirus.
It was first identified by scientists in South Africa, but cases have been found in Europe, Australia and in the UK.
Boris Johnson announced that the latest plan was for all adults to have been offered a booster jab by the end of January.
Speaking at a Downing Street press briefing, the Prime Minister said: ‘The target that we’ve set ourselves is to offer a booster to everyone eligible by the end of January.
‘As with the first jabs, we will be working through people by age group going down in five-year bands, because it is vital that the older and the more clinically vulnerable get that added protection first.
‘So, even if you have had your second jab over three months ago and you are now eligible, please don’t try and book until the NHS says it is your turn.’
The Prime Minister added: ‘We’ve already done almost 18 million boosters across the UK but we’ve got millions more to do to protect the most vulnerable.
‘Then we’ll move down the cohorts rapidly, and working together with the devolved administrations we want to ramp up capacity across the whole United Kingdom to the levels we achieved in the previous vaccination effort.
‘We’re going to be throwing everything at it in order to ensure that everyone eligible is offered that booster, as I say, in just over two months.’
Mr Johnson said he is getting his Covid-19 booster jab on Thursday.
He told a press conference: ‘I’ve been waiting patiently for my moment to come. It’s happening on Thursday I’m pleased to say.
‘So whenever your turn comes, get your booster and ask your friends and your family to do the same.
‘It’ time for another great British vaccination effort. We’ve done it before and we’re going to do it again.
‘And let’s not give this virus a second chance.’
Mr Johnson said England will have more than 1500 community pharmacy sites, adding: ‘There’ll be temporary vaccination centres popping up like Christmas trees and we’ll deploy at least 400 military personnel to assist the efforts of our NHS, alongside of course the fantastic jabs army of volunteers.
‘I know the frustration that we all feel with this Omicron variant, the sense of exhaustion that we could be going through all this all over again.
‘But today I want to stress this, today that’s the wrong thing to feel because today our position is and always will be immeasurably better than it was a year ago.
‘What we’re doing is taking some proportionate precautionary measures while our scientists crack the Omicron code.
‘And while we get the added protection of those boosters into the arms of those who need them most.’
The Prime Minister earlier defended the new rules.
‘The measures taking effect today are proportionate and responsible, and will buy us time in the face of this new (Omicron) variant,’ he said.
‘Vaccines and boosters remain our best line of defence, so it is more important than ever that people come forward when eligible to get boosted.’
Yesterday the JCVI recommended that time between the second dose and the booster jab should be cut from six months to three.
Also all adults aged 18 and over will be eligible for them.