The national booking system is now open to all adults, with people able to get a jab if it has been at least three months since they had their second vaccine.
However, bookings can be made just two months after a second dose to avoid delays.
The health service is now racing to give a booster to as many adults as possible in light of the rapidly spreading Omicron variant, after ministers warned around 200,000 new infections are being recorded per day.
The UK government is aiming to offer every eligible adult a chance to book their booster dose by 31 December as part of efforts to minimise the number of Covid-19 cases.
Dr Emily Lawson, head of the NHS Covid-19 vaccination programme for the NHS in England, said: “Millions more people can book their booster from today and NHS staff are working flat out to set up more sites and put on extra appointments.
Extra vaccination sites and mobile units are being opened in England to help meet the end of year target, and clinic opening hours extended to allow for more appointments, with centres to remain open for at least 12 hours per day over the Christmas holidays, apart from Christmas Day.
It comes after Professor Wei Shen Lim, chairman of the Covid-19 panel of the Joint Committee on Vaccination and Immunisation (JCVI), said that it was important to give the booster before the Omicron wave comes.
He told the Science and Technology Committee of MPs on Tuesday (14 December): “You want to give the booster before the wave comes, there is less benefit in giving a booster in the middle of a wave or after a wave.”
How do I book my booster jab?
Booster jabs can be booked by selecting a time and date via the NHS national booking system. The top-up dose should be given no sooner than three months after the second jab.
You should receive a letter or a text inviting you to book when you are eligible. Once you have been invited to book, you can do so online or by calling 119.
Alternatively, you can visit a walk-in centre where you do not need to have an appointment.
NHS England has advised people to use its online walk-in finder to check where their nearest vaccination site is.
Boosters are also available at walk-in centres in Scotland and Northern Ireland for those aged 30 and over.
If you are in Scotland, the NHS inform website features a handy tool which explains how you will be invited for your booster vaccination, with all over 30s now eligible to book an appointment.
In Wales, you will be contacted by your health board when it is your turn to book your jab. You should not contact your GP to ask about appointments.