The UK's Medicines & Healthcare products Regulatory Agency (MHRA) has approved the Oxford/AstraZeneca COVID-19 vaccine.
Prime Minister Boris Johnson said it was "truly fantastic news - and a triumph for British science".
The rollout begins on 4 January.
Last month the UK was first to approve the Pfizer/BioNTech vaccine and rollout of that began in the NHS this month.
In a news release, Professor Andrew Pollard, director of the Oxford Vaccine Group and chief investigator of the Oxford Vaccine Trial, said: "The regulator’s assessment that this is a safe and effective vaccine is a landmark moment, and an endorsement of the huge effort from a devoted international team of researchers and our dedicated trial participants. Though this is just the beginning, we will start to get ahead of the pandemic, protect health and economies when the vulnerable are vaccinated everywhere, as many as possible as soon as possible."
Pascal Soriot, AstraZeneca chief executive officer, added: "Today is an important day for millions of people in the UK who will get access to this new vaccine. It has been shown to be effective, well-tolerated, simple to administer and is supplied by AstraZeneca at no profit. We would like to thank our many colleagues at AstraZeneca, Oxford University, the UK government and the tens of thousands of clinical trial participants."
The MHRA was asked to begin assessment of the Oxford vaccine on November 27.
On December 9, MHRA Chief Executive Dr June Raine told MPs on the Commons Health and Science Committee that "data packages for the AstraZeneca/Oxford vaccine have been arriving" and that work was "proceeding intensively, and with great scientific rigour".
The final data were delivered to the regulator on December 23. At the time, England's Health Secretary Matt Hancock told a Downing Street briefing: "I have every confidence that the regulator will look at it with great capability. I want to put no pressure on them in terms of the decision or the timing of the decision."
Overall, the Oxford vaccine, ChAdOx1 nCoV-19, also known as AZD1222, demonstrated 70.4% protection against SARS-CoV-2, according to interim results released December 8 in The Lancet.
For participants who received two standard doses, vaccine efficacy was 62.1%.
In participants who received a low dose followed by a standard dose, efficacy was 90.0%.
The MHRA will announce the dosing regimen later.
Professor Andrew Pollard, director of the Oxford Vaccine Group, Department of Paediatrics, University of Oxford, told a news briefing on 8 December the half dose, full dose approach that gave the best results wasn't planned. He dismissed concerns about the age of participants in that group saying the result "doesn't appear to be an age phenomenon".
This weekend, AstraZeneca's Pascal Soriot told the Sunday Times: "We think we have figured out the winning formula and how to get efficacy that, after two doses, is up there with everybody else.
"I can't tell you more because we will publish at some point."
Oxford's vaccine is a chimpanzee adenovirus-vectored vaccine.
The UK has ordered 100 million doses.
Unlike the Pfizer/BioNTech vaccine that requires -70C storage, the Oxford vaccine will be easier to deliver as it can be stored at between 2C and 8C.
Other COVID-19 vaccine trials are underway in the UK, including:
Imperial vaccine trial: A phase 2 trial of LNP-nCoVsaRNA – a self-amplifying RNA vaccine developed by Imperial College (414 participants)
Novavax COVID vaccine study: A phase 3 trial of NVX-CoV2373 – a recombinant spike protein nanoparticle vaccine developed by US biotech firm, Novavax (11,955 participants)
Janssen phase 3 vaccine study: A study that opened mid-November (aims to recruit 30,000 participants worldwide)