The University of Oxford and AstraZeneca's coronavirus vaccine is 70 per cent effective, a large clinical trial shows.
The vaccine could be up to 90 per cent effective under certain conditions, AstraZeneca said.
Prime Minister Boris Johnson hailed the fantastic news but said the vaccine would still need safety checks.
On Twitter he said: "Incredibly exciting news the Oxford vaccine has proved so effective in trials.
"There are still further safety checks ahead, but these are fantastic results."
Incredibly exciting news the Oxford vaccine has proved so effective in trials. There are still further safety checks ahead, but these are fantastic results.
— Boris Johnson (@BorisJohnson) November 23, 2020
Well done to our brilliant scientists at @UniofOxford & @AstraZeneca, and all who volunteered in the trials. https://t.co/84o8TKhQga
Health Secretary Matt Hancock said the development was "really encouraging news" and said rollout could begin next year.
UK health secretary welcomes Oxford/AstraZeneca vaccine news
He told Sky News: "These figures ... show that the vaccine in the right dosage can be up to 90 per cent effective.
"We've got 100 million doses on order and should all that go well, the bulk of the rollout will be in the new year."
The vaccine stopped an average of 70 per cent of participants in the study from falling ill.
The results combined data from two different dosing regimens, which may leave questions about the best way to give the shot.
One regimen, given to 2,700 people, demonstrated efficacy of 90 per cent, while another, administered to nearly 9,000 people, showed 62 per cent efficacy.
Curiously, the best results were obtained by inoculating volunteers with a half dose of the vaccine, before using a full dose a month later.
No serious side-effects related to the vaccine were confirmed and it was well tolerated across both dosing regimens, AstraZeneca said.
Its chief executive, Pascal Soriot, said: "This vaccine's efficacy and safety confirm that it will be highly effective against Covid-19 and will have an immediate impact on this public health emergency."
The results compare well with the Moderna and Pfizer vaccines, which were 94 per cent and 95 per cent effective respectively.
The findings were reviewed after 131 trial participants contracted Covid-19, but there were no severe virus cases in the trial and no participants were admitted to hospital.
Despite the lower efficacy, the British vaccine is cheaper and easier to distribute.
While the two rivals have to be stored frozen, the AstraZeneca-Oxford shot can be kept at refrigerator temperature, which would make it easier to transport and store globally, particularly in lower and middle-income countries.
Andrew Pollard, director of the Oxford Vaccine Group, told the BBC the trial results were intriguing and more work needs to be done on the finding that an initial half dose of the shot offers more protection.
He said: "That 90 per cent is an intriguing result. So when we give a half dose as the first dose [it] means that we've got more vaccine available. And then ... after the second dose we see 90 per cent protection.
"I think that's a really exciting and intriguing result that we need to dig into further."
