Everyone aged 18 and over in England will have the chance to get their booster vaccine before the end of the year to combat the threat of rising Omicron variant cases, Boris Johnson has announced.
The prime minister said he was launching the "Omicron Emergency Booster National Mission" to encourage everyone who is eligible to "get boosted now".
The COVID booster programme was due to be opened to over 30s today, but Mr Johnson said every adult who had their second vaccine at least three months before will now be able to book a booster jab from Wednesday.
Some walk-in centres will be open from today for all adults, on the same day that work-from-home guidance is reintroduced in England as part of the government's Plan B measures.
Mr Johnson's original target was for every adult to be boosted before the end of January, but he said that is being brought forward by a month due to the Omicron variant.
Military planning teams will be brought in across the UK to support additional vaccine sites and mobile units, while clinics will extend their opening hours so they are open seven days a week, with more appointments early in the morning, in the evening and at weekends.
Thousands more volunteers will also be trained as part of the effort.
The prime minister addressed the nation after the UK's alert level was raised from three to four.
He said: "I am afraid we are now facing an emergency in our battle with the new variant, Omicron, and we must urgently reinforce our wall of vaccine protection to keep our friends and loved ones safe."
He added: "We know from bitter experience how these exponential curves develop.
"No one should be in any doubt: 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.
"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."
He said that scientists cannot currently say if Omicron is less severe, but it is much more transmissible and if the population was not boosted it would "risk a level of hospitalisation that could overwhelm our NHS and lead sadly to very many deaths".
Mr Johnson said getting all adults boosted by 1 January will require "an extraordinary effort" as he acknowledged "some other appointments will need to be postponed until the new year".
He added that "if we don't do this now, the wave of Omicron could be so big that cancellations and disruptions, like the loss of cancer appointments, would be even greater next year".
Welsh First Minister Mark Drakeford said the government in Wales is doing "everything we can" to accelerate its booster programme, with increased vaccine clinics and opening hours but warned "further steps" may be needed.
Labour leader Sir Keir Starmer said his party is in support of the accelerated booster programme announced by Mr Johnson, and also said it was "vital" eligible children get their vaccines during the Christmas holidays.
As announced last week, England is now under "Plan B" restrictions, with an increased mask mandate having already come into place last week on Friday.
From today, people must work from home if they can and from Wednesday, subject to a vote in parliament, people will need to show their NHS COVID pass to get into nightclubs and some large events.
At least 60 Tory MPs are threatening to vote against the measures, with many having a particular issue with the vaccine passports as they fear this could be extended to other places such as restaurants and bars.
The announcement was pre-recorded with no opportunity for questions, meaning the PM could not be asked about suggestions he broke COVID rules last December by attending a virtual Downing Street staff Christmas quiz.
Earlier on Sunday, the UK Health Security Agency advised ministers to raise the COVID alert level from three to four after a further 1,239 confirmed cases of the Omicron mutation were recorded in the UK as of Sunday.