The clinical trial is currently testing the Pfizer jab alongside the AstraZeneca, Moderna, Novavax, Janssen from Johnson & Johnson, Velneva and CureVac, costing £19.3 million.
Concerns regarding whether those who have had the two doses of AstraZeneca may get more benefit from a third dose of Pfizer will be investigated in the study.
The new MHRA guidance says Pfizer boosters can be given to anyone, regardless of which doses they had previously.
However, AstraZeneca boosters will only be given to those who previously had the AstraZeneca jab.
Dr June Raine, MHRA chief executive, said: “We are committed to getting safe and effective COVID-19 vaccines to the UK public.
“This means ensuring that existing COVID-19 vaccines can continue to be used in the most effective way possible.
“We know that a person’s immunity may decline over time after their first vaccine course.
“I am pleased to confirm that the COVID-19 vaccines made by Pfizer and AstraZeneca can be used as safe and effective booster doses.”
The booster shots were developed to prevent severe illness, hospitalisation, and death, even against the widely circulating Delta variant, said the Centers for Disease Control and Prevention (CDC).
However, with the Delta variant, public health experts are starting to see reduced protection against mild and moderate disease.
For that reason, booster shots were developed for vaccinated people to help maintain protection over the coming months.
“This is an important regulatory change as it gives further options for the vaccination programme, which has saved thousands of lives so far,” Dr Raine added.
“It will now be for the JVCI to advise on whether booster jabs will be given and if so, which vaccines should be used.
“We have in place a comprehensive safety surveillance strategy for monitoring the safety of all UK-approved Covid-19 vaccines and this surveillance will include booster jabs.”
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.
“Any booster programme, including which vaccines might be recommended for use, will be based the final advice of the independent JCVI.”