People eligible for a Covid-19 booster jab can book online if they have not received an invite from the NHS.

Booster doses can only be offered to people who are at least six months on from receiving their second dose of coronavirus vaccine.

Health Secretary Sajid Javid said boosters can be booked online if people have not been invited for the jab within a week of reaching the six month milestone.

Speaking at a Downing Street coronavirus press briefing, Mr Javid said: “If you’re over 50 or in another priority group and had your second jab over six months ago you’re eligible for a booster and the NHS will send you an invite.

“If you haven’t been invited within a week of reaching that six month milestone then please get on to the national booking service and book online, or phone 119, not just to save lives but to keep your freedoms too.

“Because all of these precious moments that we’ve been able to restore over the past few months, the loved ones that we’ve been able to see and the collective experiences that we’ve been able to share, they’ve been possible thanks to out vaccination programme and because of so many of you that cam forward when it was your time.

“And if we want to secure these freedoms for the long-term then the best thing that we can do is come forward once again when that moment comes.”

If eligible, people who can receive a booster dose include all adults aged 50 and over; frontline health and social care workers; and those living in residential care homes for older adults.

A total of 2.68 million people aged 80 and over in England have received two doses of vaccine, of which 1.34 million are now estimated to have had their booster dose – the equivalent of 50%.

The figures, from NHS England, also show that nearly 35% of double-jabbed people aged 75 to 79 are likely to have received a booster, along with nearly 18% of those aged 70 to 74.

In total, 4.05 million booster doses are estimated to have been delivered in England.

This is the equivalent of one in 10 of the double-jabbed population.

The Government has faced criticism for the speed of the roll-out of booster doses in England.

Professor Martin Marshall, head of the Royal College of GPs, said the vaccination situation was “concerning”, telling BBC Radio 4’s World At One: “We know that the booster vaccination is incredibly important in order to protect individual patients and indeed to protect the NHS during a winter that we expect to be really difficult, not just with Covid but with flu and other infections as well.”

Professor Andrew Hayward, an epidemiologist at University College London and a member of Scientific Advisory Group for Emergencies (Sage), said there was a need to “speed up” the rollout of vaccines.

He told Times Radio: “We are entirely reliant on the vaccination programme to reduce hospitalisations and deaths, and we know immunity from those vaccines has waned quite a lot over the course of five or six months. It is important for people if they have been offered a vaccine to take that up.”

England’s chief medical officer Professor Chris Whitty has urged people to have the vaccine, tweeting: “Covid-19 cases are rising and winter is drawing closer. 1) If you have not been vaccinated, now is the time. 2) If you are offered a booster please take up the offer. 3) Ventilation, masks in crowded indoor spaces and hand washing remain important.”

They are also being made available to people aged 16 to 49 with underlying health conditions that put them at higher risk of severe Covid-19, and adult household contacts of immunosuppressed individuals.

The booster campaign began in England on September 16, and is being delivered through existing vaccination sites including pharmacies, hospital hubs, GP practices and vaccine centres.

It is intended to provide extra protection against hospital admission or death from Covid-19 during the coming winter, and as such is being targeted at people who are more at risk from serious disease and who were jabbed during the first phase of the vaccine rollout.

Among other age groups in England, around 8% of double-jabbed people aged 60-69 are now likely to have had a booster, plus around 7% of those aged 50-59, according to analysis by the PA news agency.

Nearly 12% of double-jabbed people in north-west England, north-east England and Yorkshire have received their booster – the highest proportion for any regions.

London has the lowest proportion of any region, at around 9%.

NHS England said the figures for boosters contain a small number of third primary doses of vaccine.

These are not the same as booster doses, and are instead being offered to individuals aged 12 and over who may not be able to mount a full immune response following two doses of Covid-19 vaccination due to underlying health conditions or medical treatment.

A third primary dose of vaccine should ideally be given at least eight weeks after the second dose, according to official guidance from the Joint Committee on Vaccination and Immunisation (JCVI).