The UK has ordered 35 million more doses of the Pfizer-BioNTech vaccine, which will be delivered in the second half of 2022.
The government said it was preparing for a programme of Covid boosters to protect the most vulnerable this year.
Health and Social Care Secretary Sajid Javid said the move was intended "to future-proof our vaccine programme".
The UK has now ordered more than 540 million doses of eight different Covid vaccines.
Four have so far been approved for use in the UK.
The UK has now ordered 135 million doses of Pfizer - more than any other Covid vaccine.
The NHS was given the green light to start planning third booster doses for the over-50s, after interim advice from the Joint Committee on Vaccination and Immunisation in early July, and Mr Javid recently said the programme would start in September.
However, the UK's vaccine committee is still to give its final advice on whether a booster programme ahead of this winter should proceed and who should be included.
It is thought to be waiting for better data on how long protection from the first two doses of the vaccines lasts.
It's likely people who are immunosuppressed, with weakened immune systems, will receive a booster jab even if all over-50s - around 30 million people - do not.
Mr Javid said the UK's vaccination programme was protecting tens of millions of people from Covid-19 and saving thousands of lives.
"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," he 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."
But some scientists, including Prof Andrew Pollard, who helped develop the Oxford-AstraZeneca Vaccine, have warned that booster doses should be used to protect unvaccinated people in other countries, where they will have the greatest impact.
The UK has announced it will donate 100 million vaccine doses to developing countries, through Covax, within the next year.
The first nine million doses were donated last month.