Britain on Monday said it would take delivery of another 35 million doses of the Pfizer-BioNTech coronavirus vaccine in the second half of next year.
"While we continue to build this wall of defence from Covid-19, it's also vital we do everything we can to protect the country for the future too, whether that's from the virus as we know it or new variants," Health Minister Sajid Javid said.
"I am pleased we've reached this agreement with Pfizer for more doses as part of our robust preparations to future-proof our vaccine programme, ensuring we have plans in place to keep the nation safe for years to come."
The government has yet to decide on a possible distribution of third doses, but could begin a booster programme in early September, alongside the flu vaccine.
The Joint Committee on Vaccination and Immunisation is still assessing the data on whether boosters will be required and which groups might need them most.
Britain has been one of the countries worst hit by the virus, recording more than 131,000 deaths.
But it has embarked on a successful vaccination programme that is credited with saving about 95,000 lives, the latest data from Public Health England and the University of Cambridge shows.
Britain had already ordered 100 million doses from Pfizer, making it the country's second most used vaccine after AstraZeneca.