The Government's Joint Committee on Vaccination and Immunisation (JCVI) advised that for those between 18 and 29 the Pfizer or Moderna vaccines should be offered instead "as a preference" where those are available.
The benefits of the Oxford/AstraZeneca vaccine continue to outweigh any risks for most people, the UK medicines watchdog has said, as European regulators ruled that unusual blood clots were "very rare side effects" of the jab.
A review by the European Medicines Agency's safety committee concluded on Wednesday that "unusual blood clots with low blood platelets should be listed as very rare side effects" of the Oxford/AstraZeneca vaccine.
In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) said there were still huge benefits of the vaccine in preventing Covid-19 and serious disease.
However, due to a very small number of blood clots in younger people, those under the age of 30 will be offered Pfizer or Moderna instead.
England's deputy chief medical officer Professor Jonathan Van-Tam described the change as a "course correction". He said: "If you sail a massive liner across the Atlantic, then it's not really reasonable that you are not going to have to make at least one course correction during that voyage."
He said vaccines continued to be "the way out" for the UK and the way that the country would return to normal.
Prof Van-Tam was joined by Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), Professor Sir Munir Pirmohamed, chairman of the Commission on Human Medicines, and Professor Wei Shen, chairman of the Joint Committee on Vaccination and Immunisation (JCVI).
He said that while the balance of risk for younger people was more finely balanced, the benefits were "overwhelming" for older people who are more likely to be hospitalised with Covid.
He added: "The UK vaccine programme has been the most enormous success indeed. If you had said to me that by March 2021 we would not have needed a course correction, that also would have amazed me."
Dr Raine said the number of blood clots cases were "very rare" and the balance was "still very favourable for the vast majority of people".
The European Medicines Agency (EMA) said most of the cases of blood clots reported have occurred in women under 60 within two weeks of vaccination with the AstraZeneca jab, but that no specific risk factors had been identified based on current evidence.
Following a review of all available evidence, its Pharmacovigilance Risk Assessment Committee said the blood clots reported had been found in veins in the brain, the abdomen and arteries, combined with low levels of blood platelets and sometimes bleeding.
The EMA said symptoms associated with the blood clots include shortness of breath, chest pain, swelling in the leg, persistent abdominal pain, severe headaches, blurred vision and tiny blood spots under the skin beyond the area where the injection wad administered, and anyone who displayed them should seek medical attention.
The committee carried out an in-depth review of 62 cases of clots in the brain and 24 cases of clots in the abdomen as of March 22, with 18 of the combines cases proving fatal.
They came from reporting systems in the European Economic Area and the UK, from around 25 million people who had received the vaccine.
The European Medicines Agency reiterated that the AstraZeneca vaccine had been "proven to be highly effective" and that vaccination as a whole is "extremely important" in the fight against Covid-19.
Executive director Emer Cooke told a Brussels press briefing: "First of all, I want to start by stating that our safety committee, the Pharmacovigilance and Risk Assessment Committee (PRAC) of the European Medicines Agency, has confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risks of side effects.
"Covid-19 is a very serious disease with high hospitalisation and death rates and everyday Covid is still causing thousands of deaths across the EU.
"This vaccine has proven to be highly effective - it prevents severe disease and hospitalisation, and it is saving lives.
"Vaccination is extremely important in helping us in the fight against Covid-19 and we need to use the vaccines we have to protect us from the devastating effects.
"The PRAC, after a very in-depth analysis, has concluded that the reported cases of unusual blood clotting following vaccination with the AstraZeneca vaccine should be listed as possible side effects of the vaccine."
She said the agency's review "confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risk of side effects," adding: "Vaccination is extremely important in helping us in the fight against Covid-19."
Prime Minister Boris Johnson said he will carefully follow the advice on the Oxford/AstraZeneca vaccine from the MHRA and JCVI but does not believe he will have to alter the schedule for easing the lockdown.
During a visit to Cornwall, he told broadcasters: "I think the crucial thing on this is to listen to what the scientists, and the doctors, the medical experts, have to say.
"The MHRA is meeting, the JCVI is meeting, they'll be setting out the position and we will get on with rolling out the vaccine and obviously we'll follow very carefully what they have to say.
"I don't think anything that I have seen leads me to suppose that we will have to change the road map or deviate from the road map in any way."
It comes after Professor Adam Finn, from the University of Bristol and a member of the JCVI, said it was vital to keep the vaccine programme going as society opens up, in order to help stave off rising infection rates.
But Dr Maggie Wearmouth, a member of the JCVI, told the Daily Telegraph that "perhaps slowing things down" with the rollout "until we're absolutely certain" might be wise.
Speaking in a personal capacity, she said: "The issue is about safety and public confidence. We don't want to cover anything up that we feel that the public should be knowing.
"We're not here to blindly follow targets or due dates. We will do what is necessary."