All under-40s are to be offered an alternative to the Oxford-AstraZeneca coronavirus vaccine as a precaution.
The change in advice from the Joint Committee on Vaccination and Immunisation marks an extension to existing guidance where those aged under 30 were given a choice of COVID-19 jab over blood clotting concerns.
The Medicines and Healthcare products Regulatory Agency (MHRA) had said the balance of risk for the AstraZeneca vaccine against coronavirus is very favourable for older people but "more finely balanced" for younger groups, who do not tend to suffer serious illness with COVID.
Latest data from MHRA shows there have been 242 blood clots in combination with low platelets in more than 28 million people who had the AstraZeneca vaccine up to the 28 April.
The overall incidence of blood clots with low platelets after a first dose is put at 10.5 per million doses, and about one in a million for a second dose.
For those aged 40 to 49 the incidence is 10.1 per million doses, and 17.4 per million for those aged 30 to 39.
Overall, the death rate per million doses is 2.1, but is 4.5 for those aged 30 to 39.
The JCVI said an alternative jab should only be given where it does not cause a major delay in immunisation.
Although cases of COVID are currently low, modelling suggests a significant delay to the vaccine rollout would make a third wave more likely.
The JCVU added the AstraZeneca jab is the only one of the three licensed vaccines that can be distributed at fridge temperature, which may mean in some circumstances it is the only practical dose to offer.
People who have already had one dose of the AstraZeneca vaccine with no relevant side effects will also be offered it second time around.
Professor Wei Shen Lim, COVID-19 chairman for JCVI, said: "Safety remains our number one priority.
"We have continued to assess the benefit/risk balance of COVID-19 vaccines in light of UK infection rates and the latest information from the MHRA on the extremely rare event of blood clots and low platelet counts following vaccination.
"As COVID-19 rates continue to come under control, we are advising that adults aged 18 to 39 years with no underlying health conditions are offered an alternative to the Oxford/AstraZeneca vaccine, if available and if it does not cause delays in having the vaccine."
He added: "The COVID-19 vaccines have already saved thousands of lives and the benefit for the majority of the population is clear – if you are offered the vaccine, you should take it."
MHRA chief executive Dr June Raine said: "Our position remains that the benefits of the COVID-19 vaccine AstraZeneca against COVID-19, with its associated risk of hospitalisation and death, continues to outweigh the risks for the vast majority of people.
"The balance of benefits and risks is very favourable for older people but is more finely balanced for younger people and we advise that this evolving evidence should be taken into account when considering the use of the vaccine, as JVCI has done."
According to Public Health England (PHE), the vaccine programme is estimated to have prevented more than 10,000 deaths in England alone by the end of March.