The prime minister has pledged "temporary vaccination centres will be popping up like Christmas trees" over the coming weeks.
An expansion of the existing vaccine programme comes after UK's vaccines advisory board gave their backing to widening the rollout.
On Tuesday, the number of confirmed cases of the variant stood at at least 22 across the UK.
Boris Johnson has now set a target of offering a booster jab to everyone eligible for one by the end of January in England, in what his health secretary has called a "booster programme on steroids".
Mr Johnson said in a Downing Street press briefing on Tuesday that “we’re going to be throwing everything at it", adding military personnel will assist healthcare workers in administering booster jabs at pop-up centres.
Sajid Javid said the booster dose provides a "much higher antibody response than the primary course, so it’s more important than ever that people step up and get protected".
The Joint Committee on Vaccination and Immunisation (JCVI) earlier recommended the eligible age range should be broadened and the interval between the second and booster shots halved, in response to the recently identified Omicron variant.
Scotland, Wales and Northern Ireland are likely to announce any changes to their booster programmes in the coming days.
Who is now eligible for a booster jab in England and how do you book?
Can I now book my appointment if I'm over 18?
All adults over the age of 18 will be offered a booster shot by the end of January.
Previously, third jabs were restricted to those aged 40 and above, frontline health or social workers, and those with underlying health issues.
The new JCVI advice means those aged 18-39 will also be eligible for a booster when the NHS calls them forward.
This means more than seven million 18-39-year-olds and 6.9 million over-40s will now be eligible for a booster shot, a total of around 14 million.
When will I get my booster?
The time between second and third vaccines will be reduced to just three months in response to the Omicron variant. Previously there was a gap of six months, although you could book five months after your second jab.
The booster will be offered in order of descending age groups, with priority given to older adults and those in Covid at-risk groups.
“We will let you know when it’s your turn," said Chief Executive of the NHS Amanda Pritchard with regards to getting the jab booked.
Severely immuno suppressed people aged 16 or above who have received three primary doses will now also be offered a fourth booster dose.
How can I book?
You will be able to book your third appointment through the NHS booking service once you are eligible.
The NHS will call you forward according to eligibility.You can also get your booster dose at a walk-in Covid-19 vaccination site if you had your second dose at least six months ago.
If I've already booked, can I bring appointment forward?
As with the original booking system, it may be possible to bring your third jab appointment forward if you had booked six months after your second shot but are now eligible to have it sooner.
This may mean you have to cancel your original booking and make a new one, with a risk that you may not be able to make a new one at a time convenient to you.Will I have the same vaccine for my booster?
Not necessarily. Pfizer-BioNTech vaccine will primarily be offered as a booster dose irrespective of the vaccine used for your first two shots.
This means your booster dose may be different from the vaccines you had for your first and second doses.While most people will be offered a booster dose of the Pfizer/BioNTech vaccine, some will receive a Moderna vaccine while a few may be offered a booster dose of the Oxford/AstraZeneca vaccine if they cannot have the Pfizer/BioNTech or Moderna vaccine, the NHS said.
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Will under 18s be offered a extra jab?
The JCVI has advised second doses should be offered to 12 to 15-year-olds, 12 weeks after their first.
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), said on Monday a "through review" and monitoring had found it was safe for those aged 12 to 15 to have a second vaccine dose. She said a study of the side effects of the vaccine in those aged under 18 had shown there are "no new safety issues". Dr Raine said: "Our message to people aged 12-15 is that it is safe to have a second dose of the Pfizer vaccine and if you're called to receive your second dose, please go and take that offer. "It will ensure that you're further protected from Covid-19."
Why do I need a booster?
The latest evidence from the Scientific Advisory Group for Emergencies (Sage) shows protection against symptomatic disease falls from 65%, up to three months after the second dose, to 45% six months after the second dose for the Oxford/AstraZeneca vaccine, and from 90% to 65% for the Pfizer/BioNTech vaccine.
Protection against hospitalisation falls from 95% to 75% for Oxford/AstraZeneca and 99% to 90% for Pfizer/BioNTech.
How do booster jabs help protect against infection from the virus? Dr Sarah Jarvis explains
With fears that vaccines are less effective against the Omicron variant, getting a third shot into as many arms as possible will bolster the UK's immunity to the virus. While vaccines will still work, it is not get known if they will be less effective against this new Covid-19 variant.
Two vaccine doses provide good protection against severe illness from the original and Delta strains, but to provide better protection against infection, and transmission, three shots is ideal - and if vaccines are less effective against the Omicron variant, even more so.