The UK's medicines regulator has said the Pfizer-BioNTech and Oxford-AstraZeneca coronavirus vaccines are safe to use as booster jabs.
The MHRA issued guidance saying Pfizer boosters can be given to anyone, regardless of which doses they have had previously, while AstraZeneca boosters will only be provided to those who previously had that jab.
Britain's Joint Committee for Vaccination and Immunisation (JCVI) is examining whether booster shots are necessary for the elderly and vulnerable, with planning under way for a potential rollout this month.
Committee members are expected to decide who should get a third jab in the next few days.
Among the factors being considered are the results of the £19.3m Cov-Boost trial run by the University Hospital Southampton, which is testing the Pfizer jab along with those from AstraZeneca, Moderna, Novavax, Janssen from Johnson & Johnson, Valneva and CureVac.
The study's objectives include finding out whether people who have had two doses of the AstraZeneca vaccine may get more benefit if they have a third dose of the Pfizer jab.
Dr June Raine, the chief executive of the MHRA, said given that "a person's immunity may decline over time after their first vaccine course", the jabs made by Pfizer and AstraZeneca "can be used as safe and effective booster doses".
She said: "This is an important regulatory change as it gives further options for the vaccination programme, which has saved thousands of lives so far."
She added that "it will now be for the JVCI to advise on whether booster jabs will be given and if so, which vaccines should be used".
Yesterday the health secretary said he is "confident" that a booster programme for coronavirus jabs can begin this month.
AstraZeneca's chief executive also said booster jabs may not be needed for everyone in the UK, warning that a nationwide rollout could put additional pressures on the NHS during the winter.
A Department of Health and Social Care spokeswoman said: "We continue to prepare for an autumn booster programme to ensure those most vulnerable to COVID-19 have protection extended ahead of winter and against new variants."
Meanwhile, the head of the World Health Organisation told wealthier countries to refrain from offering booster shots until the end of the year in order to prioritise first doses for people in wealthier countries.
Tedros Adhanom Ghebreyesus said he was "appalled" by comments from pharmaceutical manufacturers who claim vaccine supplies are high enough to allow for both third jabs and vaccinations in other countries.