A third dose of the coronavirus vaccine is set to be offered those with severely weakened immune systems.

Around half a million vulnerable people will given a third jab as part of efforts to give them “the best chance of gaining protection from Covid-19".

People with conditions such as leukaemia, advanced HIV and recent organ transplants who are over the age of 12 will be eligible for another jab following a recommendation from the Joint Committee on Vaccination and Immunisation (JCVI).

On September 1, the JCVI said people who were severely immunosuppressed at the time of their first or second dose of vaccine may not have been able to mount a full response to vaccination, meaning they could be less protected than the wider population.

Charities working with people who will be eligible for a third dose welcomed the announcement as it will offer “much-needed reassurance and additional protection”.

The committee said studies are ongoing to see how effective a third dose is for this group of people, and as it is considered unlikely to cause any harm, they have decided that a third jab can be safely offered and might boost protection.

The JCVI said the timing of a third dose will be made by a patient’s specialist doctor, but that they would usually be given at least eight weeks after the second dose, with some flexibility on that.

Professor Wei Shen Lim, chair of Covid-19 Immunisation for the JCVI, said: “We want people with severely suppressed immune systems to have the best chance of gaining protection from Covid-19 via vaccination.

“Therefore, we are advising they have a third vaccine dose on top of their initial two doses, as we hope this will reduce their risk of severe outcomes such as hospitalisation and death.”

The recommendation does not apply to all those considered clinically extremely vulnerable, but is estimated to include between 400,000 and 500,000 people, or less than 1pc of the population.

Health secretary Sajid Javid said The NHS will contact people as soon as possible to arrange an appointment for a third dose.

The announcement is separate to any decision on a booster programme, but it is understood news on that is expected soon.