Boris Johnson has launched the Omicron Emergency Boost - a "national mission" to try to curb the spread of COVID infections in the UK as a "tidal wave" of the new variant "is coming".
The prime minister said the initiative will be "unlike anything we have done before in the vaccination programme" and it aims to jab almost a million people every day.
In a televised address, he warned that two doses are "not enough to give the level of protection we need" and a booster dose can "bring our level of protection back up".
"Do not make the mistake of thinking Omicron can't hurt you, can't make you or your loved ones seriously ill. We must act now," he added, saying people were already in hospital with the new variant.
Mr Johnson said that to hit the booster pace that the UK needs, it will have to match its best vaccination day so far and then beat it "day after day".
The highest number of COVID vaccinations reported in one day was 844,285 on 20 March this year, according to the government website.
On Saturday, 530,086 booster jabs or third doses were administered in the UK - the second highest daily figure since 10 November when there were 532,238.
A total of 23,124,829 people (40.2% of the population aged 12 and over in the UK) have now received a booster.
The prime minister said the national mission is needed because the number of infections of the new variant is doubling every two to three days.
How does PM plan to hit the booster target?
All those over 18 in England will have the chance to get a booster jab by the end of the year, bringing forward his original target by a month.
From Monday, the booster programme will be opened up to over-18s who have had their second dose at least three months ago.
They will be able to get their booster via walk-in sites in some areas from Monday, and from Wednesday they can use the booking system for a slot.
And he promised there will be additional support for Scotland, Wales and Northern Ireland.
Mr Johnson said he is asking those on the front line of health services for "another extraordinary effort now".
• He plans to deploy 42 military planning teams across every health region
• Additional vaccine sites and mobile units will be set up
• And opening hours will be extended so clinics can run seven days a week
• He said this will mean more appointments are available earlier in the morning, in the evening, and at weekends
• He also said thousands more volunteer vaccinators will be trained up
• The NHS will have to postpone some planned appointments in order to meet the target
The PM urged those people who have not had any vaccine to get a jab so they have some protection.
And for those who have already received their booster, he urged them to encourage friends and family to do the same.
