British adults aged between 18 and 29 will be offered an alternative to the AstraZeneca Covid-19 vaccine where possible.
UK government advisers confirmed the news following a review of the AstraZeneca jab by the health regulator there.
England's deputy chief medical officer Profess Jonathan Van-Tam said the new advice is a "course correction" for the "very successful" vaccine rollout in the UK.
The uncertainty over the vaccine has been attributed as one of the main reasons for the EU's slow vaccination programme compared to the UK.
More than 18 million people in the UK had been administered the AstraZeneca vaccine as of last weekend.
However, the UK regulator is probing whether it will be suitable for younger adults once they become eligible to receive the jab in the next few months.
It comes after concerns were raised over whether the vaccine is suitable for young people amid reports of blood clotting after a first dose.
As of March 31, 79 people in the UK have had blood clots after getting their first AstraZeneca jab.
19 of those people had died, three of whom were under the age of 30.
It also comes as the European Medicines Agency revealed its findings today as to whether the Oxford inoculation can be linked to rare blood clotting events.
The medicines regulator said that "unusual blood clots" should be listed as a "very rare" side-effect of the vaccine.
One plausible explanation for the combination of blood clots and low blood platelets is an immune response, leading to a condition similar to one seen sometimes in patients treated with heparin, the EMA said.
The agency is reminding healthcare workers and people receiving the AstraZeneca vaccine to be aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within two weeks of immunisation.
It said most of the cases reported so far have occurred in women under 60 years of age.
EU health ministers have been told the announcement would have an "immediate impact on vaccination plans" and "vaccine confidence", according to a document seen by Reuters.
