Britain will buy 60 million more doses of Pfizer-BioNTech's Covid-19 vaccine, Health Minister Matt Hancock said.
The deal more than doubles the country's supply of the vaccine before a booster programme this year.
Britain has now ordered 100 million doses of the Pfizer treatment, which is one of three Covid-19 shots being used in the country.
Mr Hancock said the doses would support a Covid-19 booster vaccination programme starting in autumn, because the biggest risk to Britain's programme was a new variant of the coronavirus.
"We're working on our plans for booster shots," he said.
"These further 60 million doses will be used alongside others as part of our booster programme from later this year, so we can protect the progress that we've all made."
Earlier on Wednesday, Public Health England said details of a booster programme were still being worked out, and it would be designed mainly for new variants.
Britain's Health Ministry said data from trials using combinations of Covid-19 vaccines would help to determine the programme.
England's deputy chief medic, Prof Jonathan Van-Tam, said that by mid to late-summer, millions of people would have had two shots of the AstraZeneca or Pfizer vaccines.
Prof Van-Tam said the question then would be whether to give a different vaccine as a booster. A study starting in June should provide answers, he said.
"We'll get an idea then of which gives you the highest boost, which perhaps gives you the broadest boost against a range of coronavirus variants, and indeed what the timings look like," Prof Van-Tam said.
"That is another reason why the vaccines taskforce has invested in contracts with not just one or two manufacturers, but six or seven so that we have that option."
Britain has administered 47.5 million vaccine doses so far, nearly 34 million of which were first shots.
It is second only to Israel in the proportion of the population who have received a first dose, which is about 50 per cent.
The UK has suffered 127,000 deaths from Covid-19, which is the fifth-highest total globally.
Britain, which has a population of 67 million, has deals for 517 million doses of eight different Covid-19 vaccines, some of which are still under development.