Britain's medical regulator on Friday (December 8) approved Moderna's COVID-19 vaccine for use, the health ministry said.
And it's agreed to purchase an additional 10 million doses as it eyed a spring rollout of the U.S.-made shot.
Three COVID-19 vaccines have now been approved for use in Britain.
The Pfizer/BioNTech jab and one developed by Oxford University and AstraZeneca are already being administered.
Britain now has 17 million doses of Moderna's vaccine on order.
Supplies will begin to be delivered to the UK from the spring once the U.S. company expands its production capability.
Britain was the first to approve Pfizer's and AstraZeneca's vaccines in a bid to get the population vaccinated quickly.
But it is behind some other major countries in giving the go-ahead to the Moderna shot.
Moderna's vaccine was 94% effective in preventing disease in late-stage clinical trials.
It has already been given regulatory approval for use in the United States, Canada, the European Union and Israel.
On Friday (December 8) the Mayor of London declared a major incident in the British capital, saying hospital beds will run out in the next few weeks unless urgent action was taken to drastically stop the spread of the disease.
The number of people in hospital is 35% higher than during the peak of the pandemic in April.