British regulators have approved a third coronavirus vaccine, but Brexit means it will not be available to administer to the first patient until the spring.

The government has agreed to buy 10m more doses of the Moderna vaccine to add to the 7m it had already ordered, after the Medicines and Healthcare products Regulatory Agency (MHRA) authorised it, the Department of Health and Social Care (DHSC) said on Friday.

“The highly effective Moderna vaccine is another impressive success for science and is another testament to the hard work of researchers and selfless clinical trial volunteers,” the deputy chief medical officer for England Prof Jonathan Van-Tam said.

“This vaccine will save lives once doses become available, but it is crucial we all continue to follow the rules to protect each other until enough people have been protected.”

The MHRA accepted the recommendation of the Commission on Human Medicines and authorised the Moderna vaccine following months of rigorous clinical trials. The vaccine has shown 94% efficacy in preventing disease, including in the elderly.

The European Medicines Agency has conditionally approved the vaccine and the first EU member states are expecting deliveries next week after the European commission ordered 160m doses. But the UK declined to benefit from the EU’s purchase agreement.

Under the deal struck by Whitehall, deliveries to the UK will not start until Moderna expands its production capability.

The Moderna jab follows the Pfizer/BioNTech and Oxford/AstraZeneca jabs in gaining such approval, with the UK becoming the first western nation to license a vaccine against Covid-19.

“This is fantastic news and another weapon in our arsenal to tame this awful disease,” the health secretary, Matt Hancock, said. “Through our vaccine delivery plan we have already vaccinated nearly 1.5 million people across the UK. The Moderna vaccine will boost our vaccination programme even further once doses become available from the spring.

“While we immunise those most at risk from Covid, I urge everyone to continue following the rules to keep cases low to protect our loved ones.”